<SEC-DOCUMENT>0001950047-23-001294.txt : 20230605
<SEC-HEADER>0001950047-23-001294.hdr.sgml : 20230605
<ACCEPTANCE-DATETIME>20230605164923
ACCESSION NUMBER:		0001950047-23-001294
CONFORMED SUBMISSION TYPE:	144
PUBLIC DOCUMENT COUNT:		1
FILED AS OF DATE:		20230605
DATE AS OF CHANGE:		20230605

SUBJECT COMPANY:	

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			Day One Biopharmaceuticals, Inc.
		CENTRAL INDEX KEY:			0001845337
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				832415215
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		144
		SEC ACT:		1933 Act
		SEC FILE NUMBER:	001-40431
		FILM NUMBER:		23993296

	BUSINESS ADDRESS:	
		STREET 1:		2000 SIERRA POINT PARKWAY, SUITE 501
		CITY:			BRISBANE
		STATE:			CA
		ZIP:			94005
		BUSINESS PHONE:		650 484-0899

	MAIL ADDRESS:	
		STREET 1:		2000 SIERRA POINT PARKWAY, SUITE 501
		CITY:			BRISBANE
		STATE:			CA
		ZIP:			94005

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	Day One Biopharmaceuticals Holding Co LLC
		DATE OF NAME CHANGE:	20210209

REPORTING-OWNER:	

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			Blackman Samuel C.
		CENTRAL INDEX KEY:			0001863676

	FILING VALUES:
		FORM TYPE:		144

	MAIL ADDRESS:	
		STREET 1:		395 OYSTER POINT BLVD.
		STREET 2:		SUITE 217
		CITY:			SOUTH SAN FRANCISCO
		STATE:			CA
		ZIP:			94080
</SEC-HEADER>
<DOCUMENT>
<TYPE>144
<SEQUENCE>1
<FILENAME>primary_doc.xml
<TEXT>
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    <headerData>
        <submissionType>144</submissionType>
        <filerInfo>
            <filer>
                <filerCredentials>
                    <cik>0001863676</cik>
                    <ccc>XXXXXXXX</ccc>
                </filerCredentials>
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            <liveTestFlag>LIVE</liveTestFlag>



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    <formData>
        <issuerInfo>
            <issuerCik>0001845337</issuerCik>
            <issuerName>DAY ONE BIOPHARMACEUTICALS INC</issuerName>
            <secFileNumber>001-40431</secFileNumber>
            <issuerAddress>
                <ns2:street1>395 Oyster Point Blvd, Suite 217</ns2:street1>
                <ns2:city>Souths San Francisco</ns2:city>
                <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                <ns2:zipCode>94080</ns2:zipCode>
            </issuerAddress>
            <issuerContactPhone>6504840899</issuerContactPhone>
            <nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>SAMUEL BLACKMAN</nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>
            <relationshipsToIssuer>
                <relationshipToIssuer>Officer</relationshipToIssuer>
            </relationshipsToIssuer>
        </issuerInfo>
        <securitiesInformation>
            <securitiesClassTitle>COMMON</securitiesClassTitle>
            <brokerOrMarketmakerDetails>
                <name>Morgan Stanley Smith Barney LLC Executive Financial Services</name>
                <address>
                    <ns2:street1>1 New York Plaza</ns2:street1>
                    <ns2:street2>38th Fl.</ns2:street2>
                    <ns2:city>New York</ns2:city>
                    <ns2:stateOrCountry>NY</ns2:stateOrCountry>
                    <ns2:zipCode>10004</ns2:zipCode>
                </address>
            </brokerOrMarketmakerDetails>
            <noOfUnitsSold>10000</noOfUnitsSold>
            <aggregateMarketValue>136000.00</aggregateMarketValue>
            <noOfUnitsOutstanding>73572633</noOfUnitsOutstanding>
            <approxSaleDate>06/05/2023</approxSaleDate>
            <securitiesExchangeName>NASDAQ</securitiesExchangeName>
        </securitiesInformation>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>05/31/2021</acquiredDate>
            <natureOfAcquisitionTransaction>Restricted Stock Units</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>10000</amountOfSecuritiesAcquired>
            <paymentDate>05/31/2021</paymentDate>
            <natureOfPayment>N/A</natureOfPayment>
        </securitiesToBeSold>
        <nothingToReportFlagOnSecuritiesSoldInPast3Months>N</nothingToReportFlagOnSecuritiesSoldInPast3Months>
        <securitiesSoldInPast3Months>
            <sellerDetails>
                <name>10b5-1 Sales Plan for SAMUEL BLACKMAN</name>
                <address>
                    <ns2:street1>395 Oyster Point Blvd, Suite 217</ns2:street1>
                    <ns2:city>Souths San Francisco</ns2:city>
                    <ns2:stateOrCountry>CA</ns2:stateOrCountry>
                    <ns2:zipCode>94080</ns2:zipCode>
                </address>
            </sellerDetails>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <saleDate>03/10/2023</saleDate>
            <amountOfSecuritiesSold>10000</amountOfSecuritiesSold>
            <grossProceeds>180484.00</grossProceeds>
        </securitiesSoldInPast3Months>
        <noticeSignature>
            <noticeDate>06/05/2023</noticeDate>
            <planAdoptionDates>
                <planAdoptionDate>08/23/2022</planAdoptionDate>
            </planAdoptionDates>
            <signature>/s/ Samuel Blackman</signature>
        </noticeSignature>
    </formData>
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