X0206 3/A 2020-06-11 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 President and CEO Stock Option (right to buy) 5.16 2030-03-04 Common Stock 199038 D Stock Option (right to buy) 5.16 2030-03-04 Common Stock 199038 D 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. This amendment is being filed to correct the exercise price of the derivative securities, which inadvertently did not give effect to the one-for-1.7663 reverse stock split that became effective on June 5, 2020. /s/ Jennifer Elliott, Attorney-in-Fact 2021-01-13