<?xml version="1.0" encoding="us-ascii"?>
<!--XBRL document created by RR Donnelley Tagger version 10.0.0.218; Copyright (C) RR Donnelley-->
<!--Based on XBRL 2.1-->
<link:linkbase xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:link="http://www.xbrl.org/2003/linkbase" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#SummaryOfSignificantAccountingPoliciesDetailsTextual" roleURI="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#FairValueOnFinancialInstrumentsDetails" roleURI="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#AvailableForSaleSecuritiesDetails" roleURI="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#AvailableForSaleSecuritiesDetails1" roleURI="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails1" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#GoodwillAndIntangibleAssetsNetDetails" roleURI="http://www.cerus.com/role/GoodwillAndIntangibleAssetsNetDetails" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#LongTermInvestmentsDetailsTextual" roleURI="http://www.cerus.com/role/LongTermInvestmentsDetailsTextual" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#DebtDetails" roleURI="http://www.cerus.com/role/DebtDetails" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#DebtDetailsTextual" roleURI="http://www.cerus.com/role/DebtDetailsTextual" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#CommitmentsAndContingenciesDetailsTextual" roleURI="http://www.cerus.com/role/CommitmentsAndContingenciesDetailsTextual" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#StockholdersEquityDetails" roleURI="http://www.cerus.com/role/StockholdersEquityDetails" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#StockholdersEquityDetails1" roleURI="http://www.cerus.com/role/StockholdersEquityDetails1" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#StockholdersEquityDetailsTextual" roleURI="http://www.cerus.com/role/StockholdersEquityDetailsTextual" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#StockBasedCompensationDetails1" roleURI="http://www.cerus.com/role/StockBasedCompensationDetails1" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#StockBasedCompensationDetailsTextual" roleURI="http://www.cerus.com/role/StockBasedCompensationDetailsTextual" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#LicenseAgreementsDetailsTextual" roleURI="http://www.cerus.com/role/LicenseAgreementsDetailsTextual" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#SegmentCustomerAndGeographicInformationDetails" roleURI="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetails" />
  <link:roleRef xlink:type="simple" xlink:href="cers-20120930.xsd#SegmentCustomerAndGeographicInformationDetailsTextual" roleURI="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetailsTextual" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" arcroleURI="http://xbrl.org/int/dim/arcrole/all" />
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_RangeAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MaximumMember" xlink:label="us-gaap_MaximumMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MinimumMember" xlink:label="us-gaap_MinimumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MinimumMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_ClassOfWarrantOrRightAxis" order="2" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_TwoThousandNineUnitOfferingMember" xlink:label="cers_TwoThousandNineUnitOfferingMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_TwoThousandTenUnitOfferingMember" xlink:label="cers_TwoThousandTenUnitOfferingMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="cers_TwoThousandNineUnitOfferingMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="cers_TwoThousandTenUnitOfferingMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_StatementClassOfStockAxis" order="3" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_WarrantMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="us-gaap_PropertyPlantAndEquipmentLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" order="4" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:PropertyPlantAndEquipmentLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="us-gaap_PropertyPlantAndEquipmentLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_SummaryOfSignificantAccountingPoliciesTextualAbstract" xlink:label="cers_SummaryOfSignificantAccountingPoliciesTextualAbstract" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_QuantificationOfImmaterialPriorPeriodAdjustmentsInPriorInterimFiscalPeriods" xlink:label="cers_QuantificationOfImmaterialPriorPeriodAdjustmentsInPriorInterimFiscalPeriods" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DeferredRevenueCurrent" xlink:label="us-gaap_DeferredRevenueCurrent" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_CumulativeOutstandingTradeReceivablesNumberOfMajorCustomers" xlink:label="cers_CumulativeOutstandingTradeReceivablesNumberOfMajorCustomers" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_CumulativeOutstandingTradeReceivablesMajorCustomersPercentage" xlink:label="cers_CumulativeOutstandingTradeReceivablesMajorCustomersPercentage" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_InventoryValuationReserves" xlink:label="us-gaap_InventoryValuationReserves" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_InventoryWriteDownAgainstWhichOffsettingWarrantyClaimAvailable" xlink:label="cers_InventoryWriteDownAgainstWhichOffsettingWarrantyClaimAvailable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:label="us-gaap_PropertyPlantAndEquipmentUsefulLife" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" xlink:label="us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ImpairmentOfLongLivedAssetsHeldForUse" xlink:label="us-gaap_ImpairmentOfLongLivedAssetsHeldForUse" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" xlink:label="us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_DerecognitionOfTaxPositions" xlink:label="cers_DerecognitionOfTaxPositions" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OpenTaxYear" xlink:label="us-gaap_OpenTaxYear" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProductWarrantyAccrual" xlink:label="us-gaap_ProductWarrantyAccrual" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ProductWarrantyReceivableCurrent" xlink:label="cers_ProductWarrantyReceivableCurrent" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_InventoryLifeFinishedGoodsPeriod" xlink:label="cers_InventoryLifeFinishedGoodsPeriod" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_InventoryLifeWorkInProcessPeriod" xlink:label="cers_InventoryLifeWorkInProcessPeriod" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_SafetyProductWarrantyPeriod" xlink:label="cers_SafetyProductWarrantyPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="cers_QuantificationOfImmaterialPriorPeriodAdjustmentsInPriorInterimFiscalPeriods" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_DeferredRevenueCurrent" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="cers_CumulativeOutstandingTradeReceivablesNumberOfMajorCustomers" order="4" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="cers_CumulativeOutstandingTradeReceivablesMajorCustomersPercentage" order="5" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_InventoryValuationReserves" order="6" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="cers_InventoryWriteDownAgainstWhichOffsettingWarrantyClaimAvailable" order="7" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentUsefulLife" order="8" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" order="9" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ImpairmentOfLongLivedAssetsHeldForUse" order="10" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" order="11" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" order="12" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="cers_DerecognitionOfTaxPositions" order="13" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_OpenTaxYear" order="14" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ProductWarrantyAccrual" order="15" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="cers_ProductWarrantyReceivableCurrent" order="16" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="cers_InventoryLifeFinishedGoodsPeriod" order="17" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="cers_InventoryLifeWorkInProcessPeriod" order="18" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="cers_SafetyProductWarrantyPeriod" order="19" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="1" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain" order="2" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual" xlink:title="06013 - Disclosure - Summary of Significant Accounting Policies (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" order="3" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="us-gaap_FairValueByLiabilityClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="us-gaap_FairValueByLiabilityClassAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="us-gaap_FairValueByLiabilityClassAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_WarrantLiabilityMember" xlink:label="cers_WarrantLiabilityMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByLiabilityClassAxis" xlink:to="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:to="cers_WarrantLiabilityMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="us-gaap_FairValueByAssetClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="us-gaap_FairValueByAssetClassAxis" order="2" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="us-gaap_FairValueByAssetClassAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="us-gaap_MoneyMarketFundsMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember" xlink:label="us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByAssetClassAxis" xlink:to="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:to="us-gaap_MoneyMarketFundsMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:to="us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" order="3" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="us-gaap_FairValueInputsLevel1Member" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="us-gaap_FairValueInputsLevel2Member" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="us-gaap_FairValueInputsLevel3Member" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel1Member" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel2Member" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel3Member" order="3" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" order="4" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisAbstract" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesFairValueDisclosure" xlink:label="us-gaap_AvailableForSaleSecuritiesFairValueDisclosure" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisObligations" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisObligations" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesFairValueDisclosure" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems" xlink:to="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisObligations" order="3" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="us-gaap_FairValueByLiabilityClassAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByLiabilityClassAxis" xlink:to="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" order="1" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="us-gaap_FairValueByAssetClassAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByAssetClassAxis" xlink:to="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" order="2" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/FairValueOnFinancialInstrumentsDetails" xlink:title="0602 - Disclosure - Fair Value on Financial Instruments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" order="3" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails" xlink:title="0603 - Disclosure - Available-for-sale Securities (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:to="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails" xlink:title="0603 - Disclosure - Available-for-sale Securities (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="us-gaap_MoneyMarketFundsMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember" xlink:label="us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:to="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:to="us-gaap_MoneyMarketFundsMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:to="us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails" xlink:title="0603 - Disclosure - Available-for-sale Securities (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:ScheduleOfAvailableForSaleSecuritiesLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails" xlink:title="0603 - Disclosure - Available-for-sale Securities (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAbstract" xlink:label="us-gaap_AvailableForSaleSecuritiesAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAmortizedCost" xlink:label="us-gaap_AvailableForSaleSecuritiesAmortizedCost" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGain" xlink:label="us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AvailableForSaleSecuritiesFairValueDisclosure" xlink:label="us-gaap_AvailableForSaleSecuritiesFairValueDisclosure" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesAmortizedCost" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGain" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesFairValueDisclosure" order="4" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails" xlink:title="0603 - Disclosure - Available-for-sale Securities (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:to="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails1" xlink:title="06031 - Disclosure - Available-for-sale Securities (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable" xlink:label="us-gaap_ScheduleOfCashAndCashEquivalentsTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis" xlink:label="us-gaap_CashAndCashEquivalentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfCashAndCashEquivalentsTable" xlink:to="us-gaap_CashAndCashEquivalentsAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails1" xlink:title="06031 - Disclosure - Available-for-sale Securities (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis" xlink:label="us-gaap_CashAndCashEquivalentsAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_CarryingValueMember" xlink:label="cers_CarryingValueMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_FairValueMember" xlink:label="cers_FairValueMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CashAndCashEquivalentsAxis" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="cers_CarryingValueMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="cers_FairValueMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails1" xlink:title="06031 - Disclosure - Available-for-sale Securities (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable" xlink:label="us-gaap_ScheduleOfCashAndCashEquivalentsTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CashAndCashEquivalentsLineItems" xlink:label="us-gaap_CashAndCashEquivalentsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_CashAndCashEquivalentsLineItems" xlink:to="us-gaap_ScheduleOfCashAndCashEquivalentsTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:CashAndCashEquivalentsLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails1" xlink:title="06031 - Disclosure - Available-for-sale Securities (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CashAndCashEquivalentsLineItems" xlink:label="us-gaap_CashAndCashEquivalentsLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_AvailableForSaleSecuritiesByContractualMaturityAbstract" xlink:label="cers_AvailableForSaleSecuritiesByContractualMaturityAbstract" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_AvailableForSaleSecuritiesByContractualMaturityDueInOneYear" xlink:label="cers_AvailableForSaleSecuritiesByContractualMaturityDueInOneYear" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_AvailableForSaleSecuritiesByContractualMaturityDueGreaterThanThreeYearsAndLessThanFiveYears" xlink:label="cers_AvailableForSaleSecuritiesByContractualMaturityDueGreaterThanThreeYearsAndLessThanFiveYears" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_AvailableForSaleSecuritiesByContractualMaturity" xlink:label="cers_AvailableForSaleSecuritiesByContractualMaturity" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CashAndCashEquivalentsLineItems" xlink:to="cers_AvailableForSaleSecuritiesByContractualMaturityDueInOneYear" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CashAndCashEquivalentsLineItems" xlink:to="cers_AvailableForSaleSecuritiesByContractualMaturityDueGreaterThanThreeYearsAndLessThanFiveYears" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CashAndCashEquivalentsLineItems" xlink:to="cers_AvailableForSaleSecuritiesByContractualMaturity" order="4" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/AvailableForSaleSecuritiesDetails1" xlink:title="06031 - Disclosure - Available-for-sale Securities (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis" xlink:label="us-gaap_CashAndCashEquivalentsAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CashAndCashEquivalentsAxis" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/GoodwillAndIntangibleAssetsNetDetails" xlink:title="0605 - Disclosure - Goodwill and Intangible Assets, net (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable" xlink:label="us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/GoodwillAndIntangibleAssetsNetDetails" xlink:title="0605 - Disclosure - Goodwill and Intangible Assets, net (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LicensingAgreementsMember" xlink:label="us-gaap_LicensingAgreementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_LicensingAgreementsMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/GoodwillAndIntangibleAssetsNetDetails" xlink:title="0605 - Disclosure - Goodwill and Intangible Assets, net (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable" xlink:label="us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:label="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:AcquiredFiniteLivedIntangibleAssetsLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/GoodwillAndIntangibleAssetsNetDetails" xlink:title="0605 - Disclosure - Goodwill and Intangible Assets, net (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:label="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract" xlink:label="us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNetAbstract" xlink:label="us-gaap_FiniteLivedIntangibleAssetsNetAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="us-gaap_FiniteLivedIntangibleAssetsGross" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="us-gaap_FiniteLivedIntangibleAssetsNet" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsGross" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" order="4" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsNet" order="5" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/GoodwillAndIntangibleAssetsNetDetails" xlink:title="0605 - Disclosure - Goodwill and Intangible Assets, net (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LongTermInvestmentsDetailsTextual" xlink:title="0606 - Disclosure - Long-Term Investments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/invest/2012/invest-2012-01-31.xsd#invest_InvestmentTable" xlink:label="invest_InvestmentTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="invest_InvestmentTable" xlink:to="us-gaap_RangeAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LongTermInvestmentsDetailsTextual" xlink:title="0606 - Disclosure - Long-Term Investments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MaximumMember" xlink:label="us-gaap_MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LongTermInvestmentsDetailsTextual" xlink:title="0606 - Disclosure - Long-Term Investments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/invest/2012/invest-2012-01-31.xsd#invest_InvestmentTable" xlink:label="invest_InvestmentTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/invest/2012/invest-2012-01-31.xsd#invest_InvestmentLineItems" xlink:label="invest_InvestmentLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="invest_InvestmentLineItems" xlink:to="invest_InvestmentTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  invest:InvestmentLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LongTermInvestmentsDetailsTextual" xlink:title="0606 - Disclosure - Long-Term Investments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/invest/2012/invest-2012-01-31.xsd#invest_InvestmentLineItems" xlink:label="invest_InvestmentLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_LongTermInvestmentsTextualAbstract" xlink:label="cers_LongTermInvestmentsTextualAbstract" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_OwnershipPercentageUnderCostMethodInvestments" xlink:label="cers_OwnershipPercentageUnderCostMethodInvestments" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="invest_InvestmentLineItems" xlink:to="cers_OwnershipPercentageUnderCostMethodInvestments" order="2" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LongTermInvestmentsDetailsTextual" xlink:title="0606 - Disclosure - Long-Term Investments (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetails" xlink:title="0608 - Disclosure - Debt (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="us-gaap_CreditFacilityAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_CreditFacilityAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetails" xlink:title="0608 - Disclosure - Debt (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="us-gaap_CreditFacilityAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="us-gaap_CreditFacilityDomain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SecuredDebtMember" xlink:label="us-gaap_SecuredDebtMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditMember" xlink:label="us-gaap_LineOfCreditMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CreditFacilityAxis" xlink:to="us-gaap_CreditFacilityDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="us-gaap_SecuredDebtMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="us-gaap_LineOfCreditMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetails" xlink:title="0608 - Disclosure - Debt (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:DebtInstrumentLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetails" xlink:title="0608 - Disclosure - Debt (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_SecuredDebtAbstract" xlink:label="cers_SecuredDebtAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentCarryingAmount" xlink:label="us-gaap_DebtInstrumentCarryingAmount" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentUnamortizedDiscount" xlink:label="us-gaap_DebtInstrumentUnamortizedDiscount" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SecuredDebt" xlink:label="us-gaap_SecuredDebt" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_DebtInstrumentCarryingCurrent" xlink:label="cers_DebtInstrumentCarryingCurrent" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_DebtInstrumentUnamortizedCurrent" xlink:label="cers_DebtInstrumentUnamortizedCurrent" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SecuredDebtCurrent" xlink:label="us-gaap_SecuredDebtCurrent" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_DebtInstrumentCarryingNonCurrent" xlink:label="cers_DebtInstrumentCarryingNonCurrent" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_DebtInstrumentUnamortizedNoncurrent" xlink:label="cers_DebtInstrumentUnamortizedNoncurrent" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SecuredLongTermDebt" xlink:label="us-gaap_SecuredLongTermDebt" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentCarryingAmount" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentUnamortizedDiscount" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_SecuredDebt" order="4" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="cers_DebtInstrumentCarryingCurrent" order="5" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="cers_DebtInstrumentUnamortizedCurrent" order="6" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_SecuredDebtCurrent" order="7" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="cers_DebtInstrumentCarryingNonCurrent" order="8" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="cers_DebtInstrumentUnamortizedNoncurrent" order="9" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_SecuredLongTermDebt" order="10" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetails" xlink:title="0608 - Disclosure - Debt (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="us-gaap_CreditFacilityAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="us-gaap_CreditFacilityDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CreditFacilityAxis" xlink:to="us-gaap_CreditFacilityDomain" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="us-gaap_LineOfCreditFacilityTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="us-gaap_CreditFacilityAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LineOfCreditFacilityTable" xlink:to="us-gaap_CreditFacilityAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="us-gaap_CreditFacilityAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="us-gaap_CreditFacilityDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_GrowthCapitalFacilityMember" xlink:label="cers_GrowthCapitalFacilityMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_GrowthCapitalLoanMember" xlink:label="cers_GrowthCapitalLoanMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ComplianceWithCovenantsMember" xlink:label="cers_ComplianceWithCovenantsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CreditFacilityAxis" xlink:to="us-gaap_CreditFacilityDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="cers_GrowthCapitalFacilityMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="cers_GrowthCapitalLoanMember" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="cers_ComplianceWithCovenantsMember" order="3" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="us-gaap_LineOfCreditFacilityTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LineOfCreditFacilityTable" xlink:to="us-gaap_FinancialInstrumentAxis" order="2" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SecuritiesPledgedAsCollateralMember" xlink:label="us-gaap_SecuritiesPledgedAsCollateralMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_SecuritiesPledgedAsCollateralMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="us-gaap_LineOfCreditFacilityTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LineOfCreditFacilityTable" xlink:to="us-gaap_DebtInstrumentAxis" order="3" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_CapitalLoanMember" xlink:label="cers_CapitalLoanMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="us-gaap_RevolvingCreditFacilityMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="cers_CapitalLoanMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="us-gaap_RevolvingCreditFacilityMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="us-gaap_LineOfCreditFacilityTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="us-gaap_LineOfCreditFacilityLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_LineOfCreditFacilityTable" order="4" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:LineOfCreditFacilityLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="us-gaap_LineOfCreditFacilityLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_DebtTextualAbstract" xlink:label="cers_DebtTextualAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:label="us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_PercentageOfInvestmentsMadeInSubsidiary" xlink:label="cers_PercentageOfInvestmentsMadeInSubsidiary" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_LineOfCreditFacilityWithdrawals" xlink:label="cers_LineOfCreditFacilityWithdrawals" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityFrequencyOfPayments" xlink:label="us-gaap_LineOfCreditFacilityFrequencyOfPayments" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentMaturityDate" xlink:label="us-gaap_DebtInstrumentMaturityDate" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityCommitmentFeeAmount" xlink:label="us-gaap_LineOfCreditFacilityCommitmentFeeAmount" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LoanProcessingFee" xlink:label="us-gaap_LoanProcessingFee" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage" xlink:label="us-gaap_DebtInstrumentInterestRateEffectivePercentage" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_FinalPaymentFeePercentage" xlink:label="cers_FinalPaymentFeePercentage" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_AmountUnderRevolvingLineOfCreditAvailableToCompany" xlink:label="cers_AmountUnderRevolvingLineOfCreditAvailableToCompany" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityAmountOutstanding" xlink:label="us-gaap_LineOfCreditFacilityAmountOutstanding" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentInterestRateTerms" xlink:label="us-gaap_DebtInstrumentInterestRateTerms" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate" xlink:label="us-gaap_DebtInstrumentBasisSpreadOnVariableRate" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd" xlink:label="us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_LineOfCreditFacilityFrequencyOfCommitmentFeePayment" xlink:label="us-gaap_LineOfCreditFacilityFrequencyOfCommitmentFeePayment" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_MinimumCashBalance" xlink:label="cers_MinimumCashBalance" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_MinimumRevenuesLevelsPercentage" xlink:label="cers_MinimumRevenuesLevelsPercentage" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SeniorNotes" xlink:label="us-gaap_SeniorNotes" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ClosingCostAndFees" xlink:label="cers_ClosingCostAndFees" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_LineOfCreditFacilityAnnualCommitmentFeeAmount" xlink:label="cers_LineOfCreditFacilityAnnualCommitmentFeeAmount" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_LineOfCreditFacilityInterestPaymentPeriod" xlink:label="cers_LineOfCreditFacilityInterestPaymentPeriod" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_LineOfCreditFacilityPrincipalAndInterestRepaymentAdditionalPeriod" xlink:label="cers_LineOfCreditFacilityPrincipalAndInterestRepaymentAdditionalPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="cers_PercentageOfInvestmentsMadeInSubsidiary" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="cers_LineOfCreditFacilityWithdrawals" order="4" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_LineOfCreditFacilityFrequencyOfPayments" order="5" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_DebtInstrumentMaturityDate" order="6" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateStatedPercentage" order="7" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_LineOfCreditFacilityCommitmentFeeAmount" order="8" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_LoanProcessingFee" order="9" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateEffectivePercentage" order="10" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="cers_FinalPaymentFeePercentage" order="11" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="cers_AmountUnderRevolvingLineOfCreditAvailableToCompany" order="12" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_LineOfCreditFacilityAmountOutstanding" order="13" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateTerms" order="14" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_DebtInstrumentBasisSpreadOnVariableRate" order="15" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd" order="16" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_LineOfCreditFacilityFrequencyOfCommitmentFeePayment" order="17" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="cers_MinimumCashBalance" order="18" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="cers_MinimumRevenuesLevelsPercentage" order="19" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="us-gaap_SeniorNotes" order="20" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="cers_ClosingCostAndFees" order="21" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="cers_LineOfCreditFacilityAnnualCommitmentFeeAmount" order="22" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="cers_LineOfCreditFacilityInterestPaymentPeriod" order="23" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LineOfCreditFacilityLineItems" xlink:to="cers_LineOfCreditFacilityPrincipalAndInterestRepaymentAdditionalPeriod" order="24" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="us-gaap_CreditFacilityAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="us-gaap_CreditFacilityDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CreditFacilityAxis" xlink:to="us-gaap_CreditFacilityDomain" order="1" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" order="2" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/DebtDetailsTextual" xlink:title="06082 - Disclosure - Debt (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" order="3" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/CommitmentsAndContingenciesDetailsTextual" xlink:title="0609 - Disclosure - Commitments and Contingencies (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfOperatingLeasedAssetsTable" xlink:label="us-gaap_ScheduleOfOperatingLeasedAssetsTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PartiesToContractualArrangementAxis" xlink:label="us-gaap_PartiesToContractualArrangementAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfOperatingLeasedAssetsTable" xlink:to="us-gaap_PartiesToContractualArrangementAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/CommitmentsAndContingenciesDetailsTextual" xlink:title="0609 - Disclosure - Commitments and Contingencies (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PartiesToContractualArrangementAxis" xlink:label="us-gaap_PartiesToContractualArrangementAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PartiesToContractualArrangementMember" xlink:label="us-gaap_PartiesToContractualArrangementMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ConcordLeaseMember" xlink:label="cers_ConcordLeaseMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PartiesToContractualArrangementAxis" xlink:to="us-gaap_PartiesToContractualArrangementMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PartiesToContractualArrangementMember" xlink:to="cers_ConcordLeaseMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/CommitmentsAndContingenciesDetailsTextual" xlink:title="0609 - Disclosure - Commitments and Contingencies (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfOperatingLeasedAssetsTable" xlink:label="us-gaap_ScheduleOfOperatingLeasedAssetsTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasedAssetsLineItems" xlink:label="us-gaap_OperatingLeasedAssetsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_OperatingLeasedAssetsLineItems" xlink:to="us-gaap_ScheduleOfOperatingLeasedAssetsTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:OperatingLeasedAssetsLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/CommitmentsAndContingenciesDetailsTextual" xlink:title="0609 - Disclosure - Commitments and Contingencies (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_OperatingLeasedAssetsLineItems" xlink:label="us-gaap_OperatingLeasedAssetsLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_CommitmentsAndContingenciesTextualAbstract" xlink:label="cers_CommitmentsAndContingenciesTextualAbstract" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_EarlyTerminationOfNonCancelableOperatingLeasesMinimumPeriod" xlink:label="cers_EarlyTerminationOfNonCancelableOperatingLeasesMinimumPeriod" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OperatingLeasedAssetsLineItems" xlink:to="cers_EarlyTerminationOfNonCancelableOperatingLeasesMinimumPeriod" order="2" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/CommitmentsAndContingenciesDetailsTextual" xlink:title="0609 - Disclosure - Commitments and Contingencies (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PartiesToContractualArrangementAxis" xlink:label="us-gaap_PartiesToContractualArrangementAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PartiesToContractualArrangementMember" xlink:label="us-gaap_PartiesToContractualArrangementMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PartiesToContractualArrangementAxis" xlink:to="us-gaap_PartiesToContractualArrangementMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetails" xlink:title="0610 - Disclosure - Stockholders'  Equity (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightTable" xlink:label="us-gaap_ClassOfWarrantOrRightTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ClassOfWarrantOrRightTable" xlink:to="us-gaap_ClassOfWarrantOrRightAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetails" xlink:title="0610 - Disclosure - Stockholders'  Equity (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_TwoThousandNineWarrantsMember" xlink:label="cers_TwoThousandNineWarrantsMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_TwoThousandTenWarrantsMember" xlink:label="cers_TwoThousandTenWarrantsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="cers_TwoThousandNineWarrantsMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="cers_TwoThousandTenWarrantsMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetails" xlink:title="0610 - Disclosure - Stockholders'  Equity (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightTable" xlink:label="us-gaap_ClassOfWarrantOrRightTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightLineItems" xlink:label="us-gaap_ClassOfWarrantOrRightLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:ClassOfWarrantOrRightLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetails" xlink:title="0610 - Disclosure - Stockholders'  Equity (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightLineItems" xlink:label="us-gaap_ClassOfWarrantOrRightLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_FairValueOfWarrantsAbstract" xlink:label="cers_FairValueOfWarrantsAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_WarrantsAndRightsOutstanding" xlink:label="us-gaap_WarrantsAndRightsOutstanding" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_WarrantsAndRightsOutstanding" order="2" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetails" xlink:title="0610 - Disclosure - Stockholders'  Equity (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetails1" xlink:title="06101 - Disclosure - Stockholders' Equity (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightTable" xlink:label="us-gaap_ClassOfWarrantOrRightTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ClassOfWarrantOrRightTable" xlink:to="us-gaap_ClassOfWarrantOrRightAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetails1" xlink:title="06101 - Disclosure - Stockholders' Equity (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_TwoThousandNineWarrantsMember" xlink:label="cers_TwoThousandNineWarrantsMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_TwoThousandTenWarrantsMember" xlink:label="cers_TwoThousandTenWarrantsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="cers_TwoThousandNineWarrantsMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="cers_TwoThousandTenWarrantsMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetails1" xlink:title="06101 - Disclosure - Stockholders' Equity (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightTable" xlink:label="us-gaap_ClassOfWarrantOrRightTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:label="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetails1" xlink:title="06101 - Disclosure - Stockholders' Equity (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:label="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssumptionsAndMethodologyForAssetsAndLiabilitiesAbstract" xlink:label="us-gaap_FairValueAssumptionsAndMethodologyForAssetsAndLiabilitiesAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssumptionsExpectedTerm" xlink:label="us-gaap_FairValueAssumptionsExpectedTerm" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssumptionsExpectedVolatilityRate" xlink:label="us-gaap_FairValueAssumptionsExpectedVolatilityRate" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssumptionsRiskFreeInterestRate" xlink:label="us-gaap_FairValueAssumptionsRiskFreeInterestRate" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssumptionsExpectedDividendRate" xlink:label="us-gaap_FairValueAssumptionsExpectedDividendRate" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_FairValueAssumptionsExpectedTerm" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_FairValueAssumptionsExpectedVolatilityRate" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_FairValueAssumptionsRiskFreeInterestRate" order="4" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_FairValueAssumptionsExpectedDividendRate" order="5" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetails1" xlink:title="06101 - Disclosure - Stockholders' Equity (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SeriesBPreferredStockMember" xlink:label="us-gaap_SeriesBPreferredStockMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_SalesAgreementMember" xlink:label="cers_SalesAgreementMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_StockholderRightsPlanMember" xlink:label="cers_StockholderRightsPlanMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesBPreferredStockMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="cers_SalesAgreementMember" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="cers_StockholderRightsPlanMember" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonStockMember" order="4" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_WarrantMember" order="5" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" order="2" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_MlvMember" xlink:label="cers_MlvMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_CantorMember" xlink:label="cers_CantorMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="cers_MlvMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="cers_CantorMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RangeAxis" order="3" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MaximumMember" xlink:label="us-gaap_MaximumMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MinimumMember" xlink:label="us-gaap_MinimumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MinimumMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_ClassOfWarrantOrRightAxis" order="4" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_TwoThousandNineUnitOfferingMember" xlink:label="cers_TwoThousandNineUnitOfferingMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_TwoThousandTenUnitOfferingMember" xlink:label="cers_TwoThousandTenUnitOfferingMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="cers_TwoThousandNineUnitOfferingMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="cers_TwoThousandTenUnitOfferingMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="us-gaap_ClassOfStockLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_StatementTable" order="5" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:ClassOfStockLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="us-gaap_ClassOfStockLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_StockholdersEquityTextualAbstract" xlink:label="cers_StockholdersEquityTextualAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="us-gaap_PreferredStockSharesIssued" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_PreferredStockConversionBasisToCommonShares" xlink:label="cers_PreferredStockConversionBasisToCommonShares" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConversionOfStockSharesIssued1" xlink:label="us-gaap_ConversionOfStockSharesIssued1" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ConversionOfStockSharesConverted1" xlink:label="us-gaap_ConversionOfStockSharesConverted1" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ConversionOfPreferredStockToCommonStockDate" xlink:label="cers_ConversionOfPreferredStockToCommonStockDate" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_PreferredStockRedemptionAmountFutureRedeemableSecurities" xlink:label="us-gaap_PreferredStockRedemptionAmountFutureRedeemableSecurities" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ExercisablePeriod" xlink:label="cers_ExercisablePeriod" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_WarrantsAndRightsOutstanding" xlink:label="us-gaap_WarrantsAndRightsOutstanding" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssumptionsRiskFreeInterestRate" xlink:label="us-gaap_FairValueAssumptionsRiskFreeInterestRate" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssumptionsExpectedTerm" xlink:label="us-gaap_FairValueAssumptionsExpectedTerm" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssumptionsExpectedDividendRate" xlink:label="us-gaap_FairValueAssumptionsExpectedDividendRate" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FairValueAssumptionsExpectedVolatilityRate" xlink:label="us-gaap_FairValueAssumptionsExpectedVolatilityRate" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_WarrantsExercised" xlink:label="cers_WarrantsExercised" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_MaximumAmountAvailableUnderOffering" xlink:label="cers_MaximumAmountAvailableUnderOffering" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_PercentageOfProceedsPayableAsCompensationToUnderwriter" xlink:label="cers_PercentageOfProceedsPayableAsCompensationToUnderwriter" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_CommonStockSold" xlink:label="cers_CommonStockSold" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_MinimumPercentageOfCommonStockAcquiredByStockholders" xlink:label="cers_MinimumPercentageOfCommonStockAcquiredByStockholders" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_DesignatedPreferredStockForFutureIssuance" xlink:label="cers_DesignatedPreferredStockForFutureIssuance" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockSharesIssued" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="cers_PreferredStockConversionBasisToCommonShares" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ConversionOfStockSharesIssued1" order="4" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ConversionOfStockSharesConverted1" order="5" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="cers_ConversionOfPreferredStockToCommonStockDate" order="6" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockRedemptionAmountFutureRedeemableSecurities" order="7" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" order="8" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights" order="9" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="cers_ExercisablePeriod" order="10" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_WarrantsAndRightsOutstanding" order="11" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_FairValueAssumptionsRiskFreeInterestRate" order="12" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_FairValueAssumptionsExpectedTerm" order="13" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_FairValueAssumptionsExpectedDividendRate" order="14" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_FairValueAssumptionsExpectedVolatilityRate" order="15" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="cers_WarrantsExercised" order="16" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="cers_MaximumAmountAvailableUnderOffering" order="17" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="cers_PercentageOfProceedsPayableAsCompensationToUnderwriter" order="18" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="cers_CommonStockSold" order="19" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="cers_MinimumPercentageOfCommonStockAcquiredByStockholders" order="20" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="cers_DesignatedPreferredStockForFutureIssuance" order="21" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ProceedsFromIssuanceOfCommonStock" order="22" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" order="1" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" order="2" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="3" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockholdersEquityDetailsTextual" xlink:title="06102 - Disclosure - Stockholders' Equity (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain" order="4" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetails1" xlink:title="06111 - Disclosure - Stock-Based Compensation (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsByReportLineAxis" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsByReportLineAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:to="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsByReportLineAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetails1" xlink:title="06111 - Disclosure - Stock-Based Compensation (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsByReportLineAxis" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsByReportLineAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsReportLineDomain" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsReportLineDomain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember" xlink:label="us-gaap_SellingGeneralAndAdministrativeExpensesMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsByReportLineAxis" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsReportLineDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsReportLineDomain" xlink:to="us-gaap_ResearchAndDevelopmentExpenseMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsReportLineDomain" xlink:to="us-gaap_SellingGeneralAndAdministrativeExpensesMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetails1" xlink:title="06111 - Disclosure - Stock-Based Compensation (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetails1" xlink:title="06111 - Disclosure - Stock-Based Compensation (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract" xlink:label="us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" order="2" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetails1" xlink:title="06111 - Disclosure - Stock-Based Compensation (Details 1)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsByReportLineAxis" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsByReportLineAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsReportLineDomain" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsReportLineDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsByReportLineAxis" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsReportLineDomain" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetailsTextual" xlink:title="06112 - Disclosure - Stock-Based Compensation (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_RangeAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetailsTextual" xlink:title="06112 - Disclosure - Stock-Based Compensation (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MaximumMember" xlink:label="us-gaap_MaximumMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetailsTextual" xlink:title="06112 - Disclosure - Stock-Based Compensation (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" order="2" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetailsTextual" xlink:title="06112 - Disclosure - Stock-Based Compensation (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_TwoThousandEightEquityIncentivePlanMember" xlink:label="cers_TwoThousandEightEquityIncentivePlanMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_EmployeeStockPurchasePlanMember" xlink:label="cers_EmployeeStockPurchasePlanMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:to="cers_TwoThousandEightEquityIncentivePlanMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:to="cers_EmployeeStockPurchasePlanMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetailsTextual" xlink:title="06112 - Disclosure - Stock-Based Compensation (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" order="3" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetailsTextual" xlink:title="06112 - Disclosure - Stock-Based Compensation (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_StockBasedCompensationTextualAbstract" xlink:label="cers_StockBasedCompensationTextualAbstract" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_IncreaseInSharesOfCommonStockAuthorizedForIssuance" xlink:label="cers_IncreaseInSharesOfCommonStockAuthorizedForIssuance" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_MaximumNumberOfSharesOfCommonStockAuthorizedForIssuance" xlink:label="cers_MaximumNumberOfSharesOfCommonStockAuthorizedForIssuance" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_NumberOfSharesAvailableForFutureIssuance" xlink:label="cers_NumberOfSharesAvailableForFutureIssuance" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance" xlink:label="us-gaap_CommonStockCapitalSharesReservedForFutureIssuance" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_OptionsAndOtherStockBasedAwards" xlink:label="cers_OptionsAndOtherStockBasedAwards" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_PerformanceBasedStockOptionsOutstanding" xlink:label="cers_PerformanceBasedStockOptionsOutstanding" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_StockBasedCompensationAssociatedWithPerformanceBasedStockOptions" xlink:label="cers_StockBasedCompensationAssociatedWithPerformanceBasedStockOptions" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRights" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRights" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="cers_IncreaseInSharesOfCommonStockAuthorizedForIssuance" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="cers_MaximumNumberOfSharesOfCommonStockAuthorizedForIssuance" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="cers_NumberOfSharesAvailableForFutureIssuance" order="4" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_CommonStockCapitalSharesReservedForFutureIssuance" order="5" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="cers_OptionsAndOtherStockBasedAwards" order="6" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="cers_PerformanceBasedStockOptionsOutstanding" order="7" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="cers_StockBasedCompensationAssociatedWithPerformanceBasedStockOptions" order="8" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRights" order="9" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetailsTextual" xlink:title="06112 - Disclosure - Stock-Based Compensation (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" order="1" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/StockBasedCompensationDetailsTextual" xlink:title="06112 - Disclosure - Stock-Based Compensation (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LicenseAgreementsDetailsTextual" xlink:title="0612 - Disclosure - License Agreements (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="dei_LegalEntityAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LicenseAgreementsDetailsTextual" xlink:title="0612 - Disclosure - License Agreements (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ZeroPointOneOfProductSalesForPlateletSystemMember" xlink:label="cers_ZeroPointOneOfProductSalesForPlateletSystemMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ZeroPointZeroThreeOfProductSalesForPlasmaSystemMember" xlink:label="cers_ZeroPointZeroThreeOfProductSalesForPlasmaSystemMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ZeroPointZeroFiveOfProductSalesForRedBloodCellSystemMember" xlink:label="cers_ZeroPointZeroFiveOfProductSalesForRedBloodCellSystemMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ZeroPointZeroSixFiveOnSaleOfIlluminatorsMember" xlink:label="cers_ZeroPointZeroSixFiveOnSaleOfIlluminatorsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="cers_ZeroPointOneOfProductSalesForPlateletSystemMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="cers_ZeroPointZeroThreeOfProductSalesForPlasmaSystemMember" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="cers_ZeroPointZeroFiveOfProductSalesForRedBloodCellSystemMember" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="cers_ZeroPointZeroSixFiveOnSaleOfIlluminatorsMember" order="4" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LicenseAgreementsDetailsTextual" xlink:title="0612 - Disclosure - License Agreements (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" order="2" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LicenseAgreementsDetailsTextual" xlink:title="0612 - Disclosure - License Agreements (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RoyaltyAgreementsMember" xlink:label="us-gaap_RoyaltyAgreementsMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_ManufacturingAndSupplyAgreementsMember" xlink:label="cers_ManufacturingAndSupplyAgreementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_RoyaltyAgreementsMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="cers_ManufacturingAndSupplyAgreementsMember" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LicenseAgreementsDetailsTextual" xlink:title="0612 - Disclosure - License Agreements (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="us-gaap_RelatedPartyTransactionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="3" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:RelatedPartyTransactionLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LicenseAgreementsDetailsTextual" xlink:title="0612 - Disclosure - License Agreements (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="us-gaap_RelatedPartyTransactionLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_LicenseAgreementTextualAbstract" xlink:label="cers_LicenseAgreementTextualAbstract" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_RoyaltyRateAppliedTowardsSaleOfProducts" xlink:label="cers_RoyaltyRateAppliedTowardsSaleOfProducts" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_AccountsPayableCurrentAndNoncurrent" xlink:label="us-gaap_AccountsPayableCurrentAndNoncurrent" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="cers_RoyaltyRateAppliedTowardsSaleOfProducts" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_AccountsPayableCurrentAndNoncurrent" order="3" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LicenseAgreementsDetailsTextual" xlink:title="0612 - Disclosure - License Agreements (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2012/dei-2012-01-31.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" order="1" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/LicenseAgreementsDetailsTextual" xlink:title="0612 - Disclosure - License Agreements (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" order="2" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetails" xlink:title="0613 - Disclosure - Segment, Customer and Geographic Information (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" xlink:label="us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorCustomersAxis" xlink:label="us-gaap_MajorCustomersAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" xlink:to="us-gaap_MajorCustomersAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetails" xlink:title="0613 - Disclosure - Segment, Customer and Geographic Information (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorCustomersAxis" xlink:label="us-gaap_MajorCustomersAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NameOfMajorCustomerDomain" xlink:label="us-gaap_NameOfMajorCustomerDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_EtablissementFrancaisDuSangMember" xlink:label="cers_EtablissementFrancaisDuSangMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_MovacoSaMember" xlink:label="cers_MovacoSaMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_AumLtdMember" xlink:label="cers_AumLtdMember" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_DelrusIncMember" xlink:label="cers_DelrusIncMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_MajorCustomersAxis" xlink:to="us-gaap_NameOfMajorCustomerDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NameOfMajorCustomerDomain" xlink:to="cers_EtablissementFrancaisDuSangMember" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NameOfMajorCustomerDomain" xlink:to="cers_MovacoSaMember" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NameOfMajorCustomerDomain" xlink:to="cers_AumLtdMember" order="3" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NameOfMajorCustomerDomain" xlink:to="cers_DelrusIncMember" order="4" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetails" xlink:title="0613 - Disclosure - Segment, Customer and Geographic Information (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" xlink:label="us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:label="us-gaap_EntityWideRevenueMajorCustomerLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:to="us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:EntityWideRevenueMajorCustomerLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetails" xlink:title="0613 - Disclosure - Segment, Customer and Geographic Information (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:label="us-gaap_EntityWideRevenueMajorCustomerLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_CustomersThatAccountedForMoreThanTenPercentageOfCompanyRevenueAbstract" xlink:label="cers_CustomersThatAccountedForMoreThanTenPercentageOfCompanyRevenueAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EntityWideRevenueMajorCustomerPercentage" xlink:label="us-gaap_EntityWideRevenueMajorCustomerPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:to="us-gaap_EntityWideRevenueMajorCustomerPercentage" order="2" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetails" xlink:title="0613 - Disclosure - Segment, Customer and Geographic Information (Details)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorCustomersAxis" xlink:label="us-gaap_MajorCustomersAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NameOfMajorCustomerDomain" xlink:label="us-gaap_NameOfMajorCustomerDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_MajorCustomersAxis" xlink:to="us-gaap_NameOfMajorCustomerDomain" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (Line Items to Hypercube)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetailsTextual" xlink:title="06131 - Disclosure - Segment, Customer and Geographic Information (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" xlink:label="us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorCustomersAxis" xlink:label="us-gaap_MajorCustomersAxis" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" xlink:to="us-gaap_MajorCustomersAxis" order="1" />
  </link:definitionLink>
  <!-- Domain Members -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetailsTextual" xlink:title="06131 - Disclosure - Segment, Customer and Geographic Information (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorCustomersAxis" xlink:label="us-gaap_MajorCustomersAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NameOfMajorCustomerDomain" xlink:label="us-gaap_NameOfMajorCustomerDomain" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_GovernmentGrantsAndCooperativeAgreementsMember" xlink:label="cers_GovernmentGrantsAndCooperativeAgreementsMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_MajorCustomersAxis" xlink:to="us-gaap_NameOfMajorCustomerDomain" order="1" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NameOfMajorCustomerDomain" xlink:to="cers_GovernmentGrantsAndCooperativeAgreementsMember" order="1" />
  </link:definitionLink>
  <!-- Hypercube Definition (All)-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetailsTextual" xlink:title="06131 - Disclosure - Segment, Customer and Geographic Information (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" xlink:label="us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:label="us-gaap_EntityWideRevenueMajorCustomerLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:to="us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
  </link:definitionLink>
  <!-- Inheritance for Line Items:  us-gaap:EntityWideRevenueMajorCustomerLineItems-->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetailsTextual" xlink:title="06131 - Disclosure - Segment, Customer and Geographic Information (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:label="us-gaap_EntityWideRevenueMajorCustomerLineItems" />
    <link:loc xlink:type="locator" xlink:href="cers-20120930.xsd#cers_SegmentCustomerAndGeographicInformationTextualAbstract" xlink:label="cers_SegmentCustomerAndGeographicInformationTextualAbstract" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_EntityWideRevenueMajorCustomerPercentage" xlink:label="us-gaap_EntityWideRevenueMajorCustomerPercentage" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_RevenueFromGrants" xlink:label="us-gaap_RevenueFromGrants" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:to="us-gaap_EntityWideRevenueMajorCustomerPercentage" order="2" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:to="us-gaap_RevenueFromGrants" order="3" />
  </link:definitionLink>
  <!-- Dimension Defaults -->
  <link:definitionLink xlink:type="extended" xlink:role="http://www.cerus.com/role/SegmentCustomerAndGeographicInformationDetailsTextual" xlink:title="06131 - Disclosure - Segment, Customer and Geographic Information (Details Textual)">
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_MajorCustomersAxis" xlink:label="us-gaap_MajorCustomersAxis" />
    <link:loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2012/elts/us-gaap-2012-01-31.xsd#us-gaap_NameOfMajorCustomerDomain" xlink:label="us-gaap_NameOfMajorCustomerDomain" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_MajorCustomersAxis" xlink:to="us-gaap_NameOfMajorCustomerDomain" order="1" />
  </link:definitionLink>
</link:linkbase>
