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  <link:definitionLink xlink:type="extended" xlink:role="http://passagebio.com/role/LeasesFyDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionLineItems" xlink:label="LesseeLeaseDescriptionLineItems" xlink:title="LesseeLeaseDescriptionLineItems" />
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    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LesseeLeaseDescriptionLineItems" xlink:to="AreaSubleased" xlink:title="definition: LesseeLeaseDescriptionLineItems to AreaSubleased" order="1.0" />
    <link:loc xlink:type="locator" xlink:href="pasg-20260721.xsd#pasg_AreaOfLandSubleasedPercentage" xlink:label="AreaOfLandSubleasedPercentage" xlink:title="AreaOfLandSubleasedPercentage" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LesseeLeaseDescriptionLineItems" xlink:to="AreaOfLandSubleasedPercentage" xlink:title="definition: LesseeLeaseDescriptionLineItems to AreaOfLandSubleasedPercentage" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AreaOfLand" xlink:label="AreaOfLand" xlink:title="AreaOfLand" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LesseeLeaseDescriptionLineItems" xlink:to="AreaOfLand" xlink:title="definition: LesseeLeaseDescriptionLineItems to AreaOfLand" order="3.0" />
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    <link:loc xlink:type="locator" xlink:href="pasg-20260721.xsd#pasg_NewLeaseAgreementOperatingLeaseNumberOfExtensions" xlink:label="NewLeaseAgreementOperatingLeaseNumberOfExtensions" xlink:title="NewLeaseAgreementOperatingLeaseNumberOfExtensions" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LesseeLeaseDescriptionLineItems" xlink:to="NewLeaseAgreementOperatingLeaseNumberOfExtensions" xlink:title="definition: LesseeLeaseDescriptionLineItems to NewLeaseAgreementOperatingLeaseNumberOfExtensions" order="5.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseRemainingLeaseTerm" xlink:label="LesseeOperatingLeaseRemainingLeaseTerm" xlink:title="LesseeOperatingLeaseRemainingLeaseTerm" />
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    <link:loc xlink:type="locator" xlink:href="pasg-20260721.xsd#pasg_LesseeOperatingLeaseNoticePeriodForTerminationOfLease" xlink:label="LesseeOperatingLeaseNoticePeriodForTerminationOfLease" xlink:title="LesseeOperatingLeaseNoticePeriodForTerminationOfLease" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LesseeLeaseDescriptionLineItems" xlink:to="LesseeOperatingLeaseNoticePeriodForTerminationOfLease" xlink:title="definition: LesseeLeaseDescriptionLineItems to LesseeOperatingLeaseNoticePeriodForTerminationOfLease" order="8.0" />
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    <link:loc xlink:type="locator" xlink:href="pasg-20260721.xsd#pasg_PercentageOfAnnualIncreaseInSubleaseRent" xlink:label="PercentageOfAnnualIncreaseInSubleaseRent" xlink:title="PercentageOfAnnualIncreaseInSubleaseRent" />
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    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LesseeLeaseDescriptionLineItems" xlink:to="RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent" xlink:title="definition: LesseeLeaseDescriptionLineItems to RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent" order="11.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseTermOfContract" xlink:label="LesseeOperatingLeaseTermOfContract" xlink:title="LesseeOperatingLeaseTermOfContract" />
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    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LesseeLeaseDescriptionLineItems" xlink:to="LesseeOperatingLeaseLiabilityPaymentsDueYearThree" xlink:title="definition: LesseeLeaseDescriptionLineItems to LesseeOperatingLeaseLiabilityPaymentsDueYearThree" order="22.0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueYearFour" xlink:label="LesseeOperatingLeaseLiabilityPaymentsDueYearFour" xlink:title="LesseeOperatingLeaseLiabilityPaymentsDueYearFour" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="LesseeLeaseDescriptionLineItems" xlink:to="LesseeOperatingLeaseLiabilityPaymentsDueYearFour" xlink:title="definition: LesseeLeaseDescriptionLineItems to LesseeOperatingLeaseLiabilityPaymentsDueYearFour" order="23.0" />
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    <link:loc xlink:type="locator" xlink:href="pasg-20260721.xsd#pasg_OperatingLeasesForOfficeSpaceMember" xlink:label="OperatingLeasesForOfficeSpaceMember" xlink:title="OperatingLeasesForOfficeSpaceMember" />
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    <link:loc xlink:type="locator" xlink:href="pasg-20260721.xsd#pasg_SubleaseAgreementaMember" xlink:label="SubleaseAgreementaMember" xlink:title="SubleaseAgreementaMember" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="PropertyPlantAndEquipmentTypeDomain" xlink:to="SubleaseAgreementaMember" xlink:title="definition: PropertyPlantAndEquipmentTypeDomain to SubleaseAgreementaMember" order="2.0" />
    <link:loc xlink:type="locator" xlink:href="pasg-20260721.xsd#pasg_SubleaseAgreementBMember" xlink:label="SubleaseAgreementBMember" xlink:title="SubleaseAgreementBMember" />
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