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Preliminary Proxy Statement

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Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

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Definitive Proxy Statement

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Definitive Additional Materials

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Fee paid previously with preliminary materials.

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Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11.

Your Vote Counts! GENERATION BIO CO. 301 BINNEY STREET CAMBRIDGE, MA 02142 GENERATION BIO CO. 2023 Annual Meeting Vote by June 7, 2023 11:59 PM ET You invested in GENERATION BIO CO. and it’s time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy material for the stockholder meeting to be held on June 8, 2023. Vote Virtually at the Meeting* June 8, 2023 9:00 AM, ET Virtually at: www.virtualshareholdermeeting.com/GBIO2023 *Please check the meeting materials for any special requirements for meeting attendance. Smartphone users Point your camera here and vote without entering a control number V1.1 For complete information and to vote, visit www.ProxyVote.com Control # V00033-P90060 Get informed before you vote View the Notice and Proxy Statement and Annual Report online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 25, 2023. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy.

THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Vote at www.ProxyVote.com Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Delivery Settings”. Voting Items Board Recommends V00034-P90060 01) Dannielle Appelhans 02) Gustav Christensen 03) Charles Rowland 04) Catherine Stehman-Breen 1. Election of four Class III directors to our board of directors, each to serve until the 2026 Annual Meeting of Stockholders. Nominees: 2. Ratification of the appointment of Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2023. 3. Approval of an amendment to our restated certificate of incorporation to reflect new Delaware law provisions regarding officer exculpation. 4. Approval, on an advisory (non-binding) basis, of the compensation of our named executive officers. NOTE: In their discretion, the proxyholders are authorized to vote upon such other business as may properly come before the meeting and any adjournment or postponement thereof. For For For For