<SEC-DOCUMENT>0001209191-21-026747.txt : 20210414
<SEC-HEADER>0001209191-21-026747.hdr.sgml : 20210414
<ACCEPTANCE-DATETIME>20210414201904
ACCESSION NUMBER:		0001209191-21-026747
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20210412
FILED AS OF DATE:		20210414
DATE AS OF CHANGE:		20210414

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Atlas Venture Associates X, LLC
		CENTRAL INDEX KEY:			0001742765
		STATE OF INCORPORATION:			DE

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-39319
		FILM NUMBER:		21827007

	BUSINESS ADDRESS:	
		STREET 1:		400 TECH. SQ.
		STREET 2:		10TH FLOOR
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02139
		BUSINESS PHONE:		857-201-2700

	MAIL ADDRESS:	
		STREET 1:		400 TECH. SQ.
		STREET 2:		10TH FLOOR
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02139

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			ATLAS VENTURE ASSOCIATES X, L.P.
		CENTRAL INDEX KEY:			0001742766
		STATE OF INCORPORATION:			DE

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-39319
		FILM NUMBER:		21827008

	BUSINESS ADDRESS:	
		STREET 1:		400 TECHNOLOGY SQ.
		STREET 2:		10TH FLOOR
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02139
		BUSINESS PHONE:		857-201-2700

	MAIL ADDRESS:	
		STREET 1:		400 TECHNOLOGY SQ.
		STREET 2:		10TH FLOOR
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02139

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			Generation Bio Co.
		CENTRAL INDEX KEY:			0001733294
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				814301281
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		301 BINNEY STREET
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02142
		BUSINESS PHONE:		857-529-5908

	MAIL ADDRESS:	
		STREET 1:		301 BINNEY STREET
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02142
</SEC-HEADER>
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<TYPE>4
<SEQUENCE>1
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<DESCRIPTION>FORM 4 SUBMISSION
<TEXT>
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        <issuerCik>0001733294</issuerCik>
        <issuerName>Generation Bio Co.</issuerName>
        <issuerTradingSymbol>GBIO</issuerTradingSymbol>
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        <reportingOwnerAddress>
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            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CAMBRIDGE</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02139</rptOwnerZipCode>
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    </reportingOwner>

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                </transactionShares>
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                    <footnoteId id="F5"/>
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                <transactionAcquiredDisposedCode>
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                <sharesOwnedFollowingTransaction>
                    <value>30767</value>
                </sharesOwnedFollowingTransaction>
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        <nonDerivativeTransaction>
            <securityTitle>
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            <transactionDate>
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                <footnoteId id="F1"/>
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            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>7354</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>25.53</value>
                    <footnoteId id="F6"/>
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                <sharesOwnedFollowingTransaction>
                    <value>23413</value>
                </sharesOwnedFollowingTransaction>
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                <directOrIndirectOwnership>
                    <value>D</value>
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        </nonDerivativeTransaction>
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            </transactionTimeliness>
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                <transactionShares>
                    <value>10539</value>
                </transactionShares>
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                <sharesOwnedFollowingTransaction>
                    <value>12874</value>
                </sharesOwnedFollowingTransaction>
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        </nonDerivativeTransaction>
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                    <value>26.78</value>
                    <footnoteId id="F8"/>
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                    <value>D</value>
                </transactionAcquiredDisposedCode>
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            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
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                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F3"/>
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        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9111939</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote</value>
                    <footnoteId id="F9"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Shares were sold pursuant to a Rule 10b5-1 trading plan dated January 14, 2021.</footnote>
        <footnote id="F2">The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $25.405 to $26.40 inclusive. The Reporting Persons undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in footnotes (2), (4), (5), (6), (7) and (8).</footnote>
        <footnote id="F3">The shares are held directly by Atlas Venture Associates X, L.P. (&quot;AVA X LP&quot;). Atlas Venture Associates X, LLC (&quot;AVA X LLC&quot;) is the general partner of AVA X LP. AVA X LLC disclaims Section 16 beneficial ownership of the securities held by AVA X LP except to the extent of its pecuniary interest therein, if any.</footnote>
        <footnote id="F4">The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $26.41 to $27.20 inclusive.</footnote>
        <footnote id="F5">The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $24.37 to $25.36 inclusive.</footnote>
        <footnote id="F6">The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $25.37 to $26.04 inclusive.</footnote>
        <footnote id="F7">The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $25.58 to $26.57 inclusive.</footnote>
        <footnote id="F8">The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $26.58 to $27.14 inclusive.</footnote>
        <footnote id="F9">These shares are held directly by Atlas Venture Fund X, L.P. (&quot;Atlas Venture Fund X&quot;). The general partner of Atlas Venture Fund X is AVA X LP and the general partner of AVA X LP is AVA X LLC. Each of AVA X LP and AVA LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Fund X, except to the extend of its respective pecuniary interest therein, if any.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Atlas Venture Associates X, L.P., By: Atlas Venture Associates X, LLC, Its: General Partner, By: /s/ Ommer Chohan, Chief Financial Officer</signatureName>
        <signatureDate>2021-04-14</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Atlas Venture Associates X, LLC, By: /s/ Ommer Chohan, Chief Financial Officer</signatureName>
        <signatureDate>2021-04-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
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</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
