X0206 3 2020-06-11 0 0001733294 Generation Bio Co. GBIO 0001732701 MCDONOUGH GEOFF C/O GENERATION BIO CO., 301 BINNEY STREET CAMBRIDGE MA 02142 1 1 1 0 See Remarks Common Stock 1365849 D Common Stock 226462 I See Footnote Series A Preferred Stock Common Stock 113231 D Stock Option (right to buy) 2.92 2030-03-04 Common Stock 199038 D Stock Option (right to buy) 2.92 2030-03-04 Common Stock 199038 D The securities are directly held by the McDonough Family 2018 Irrevocable Trust, or the Trust. The reporting person is the settlor of the Trust, Allison L. McDonough and David S. Grayzel are trustees for the Trust and the Trust is for the benefit of Graeme and Owen McDonough. The reporting person may be deemed to beneficially own the shares of common stock owned by the Trust. The Series A Preferred Stock automatically converts into the number of shares of Common Stock as shown in Column 3 without payment of further consideration at the holder's election or upon closing of the initial public offering of the Issuer's common stock. The shares have no expiration date. The option was granted on March 5, 2020. The shares underlying the option vest in full upon the date on which the Board of Directors of the Issuer determines that the Issuer has dosed the initial patient in its first clinical trial, provided that such event occurs on or before July 1, 2023. The option was granted on March 5, 2020. The shares underlying the option vest over four years, with 25% of the shares vesting on March 5, 2021 and the remaining shares vesting in equal quarterly installments thereafter. President and Chief Executive Officer Exhibit Index: 24.1 Power of Attorney /s/ Jennifer Elliott, Attorney-in-Fact 2020-06-11