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The sales reported on this Form 4 represent the number of shares required to be sold by the Reporting Person to cover tax withholding obligations in connection with the vesting of restricted stock units. Such sales were automatic and not at the discretion of the Reporting Person."},{"field":"balance_after","element":"sharesOwnedFollowingTransaction","label":"F2","text":"Includes 1,405 shares of Common Stock previously acquired under the Issuer's 2024 Employee Stock Purchase Plan."}],"balance_after":33197,"source":{"system":"sec-edgar","filing":"0001193125-26-274121","document":"ownership.xml","document_sequence":1,"table":"nonDerivative","row_type":"transaction","position":1,"url":"https://www.sec.gov/Archives/edgar/data/2031693/000119312526274121/ownership.xml","form_type":"4"}},{"id":838651,"issuer":{"id":5743,"kind":"organization","name":"Upstream Bio, Inc.","symbol":"UPB"},"owners":[{"id":37344,"kind":"person","name":"Deykin Aaron","position":1,"cik":"0002031693","reported_name":"Deykin Aaron","relationship":{"director":false,"officer":true,"ten_percent_owner":false,"other":false,"officer_title":"Chief Medical Officer"}}],"security":"Common Stock","published_at":"2026-03-17T21:02:57.000Z","date":"2026-03-16","kind":"sale","planned":false,"acquired":false,"shares":894,"price":9.29,"value":8305.26,"ownership":{"direct":true},"footnotes":[{"field":"kind","element":"transactionCoding","label":"F1","text":"The Issuer has adopted a \"sell-to-cover\" policy to satisfy the tax withholding obligations of the Reporting Person. 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