Generation Bio Co.

Item 3.03 Material Modification to Rights of Security Holders. To the extent required by Item 3.03 of Form 8-K, the information contained in Item 5.03 of this Current Report on Form 8-K is incorporated herein by reference.

Item 5.03 Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year. On July 18, 2025, Generation Bio Co. (the “Company”) filed a Certificate of Amendment to the Company’s Amended and Restated Certificate of Incorporation (the “Certificate of Amendment”) with the Secretary of State of the State of Delaware, which will effect, as of 5:00 p.m. Eastern Time, on July 21, 2025 (the “Effective Time”), a 1-for-10 reverse stock split (the “Reverse Stock Split”) of the issued and outstanding shares of the Company’s common stock, $0.0001 par value per share (the “Common Stock”). The Reverse Stock Split is intended to, among other things, bring the Company into compliance with the minimum bid price requirement for continued listing on the Nasdaq Global Select Market, as previously reported in the Company’s definitive proxy statement for the Company’s annual meeting of stockholders held on June 4, 2025, as filed with the Securities and Exchange Commission on April 23, 2025 (the “Proxy Statement”). At the Effective Time, every 10 shares of issued and outstanding Common Stock will be automatically reclassified and combined into one share of Common Stock. No fractional shares will be issued in connection with the Reverse Stock Split. Stockholders who would otherwise be entitled to a fractional share of Common Stock are instead entitled to receive a cash payment in lieu of such fractional shares. The Reverse Stock Split will not change the number of authorized shares of Common Stock or the par value of the Common Stock. Proportionate adjustments will be made to the number of shares of Common Stock available for issuance under the Company’s equity incentive plans and the number of shares underlying, and the exercise prices of, outstanding equity awards under such plans in accordance with their respective terms and as described in the Proxy Statement. The Common Stock is expected to begin trading on a post-Reverse Stock Split basis at the market open on July 22, 2025 under the Company’s existing trading symbol “GBIO”. The new CUSIP number for the Common Stock following the Reverse Stock Split is 37148K 209. The foregoing description of the Certificate of Amendment does not purport to be complete and is qualified in its entirety by reference to the complete text of the Certificate of Amendment, a copy of which is attached hereto as Exhibit 3.1 and is incorporated herein by reference.