X0306 4 2020-08-14 0 0001733294 Generation Bio Co. GBIO 0001814308 Angelino Mark D. C/O GENERATION BIO CO., 301 BINNEY STREET CAMBRIDGE MA 02142 0 1 0 0 Chief Operating Officer Common Stock 2020-08-14 4 M 0 62560 0.60 A 345637 D Common Stock 169846 I See footnote Stock Option (right to buy) 0.60 2020-08-14 4 M 0 62560 0.00 D 2028-01-04 Common Stock 62560 37536 D The securities are directly held by the Mark D. Angelino 2020 Qualified Annuity Trust, or the Trust. Mark D. Angelino is the settlor and trustee of the Trust, and the Trust is for the benefit of Elizabeth I., Emily A., John P., and Matthew R. Angelino. The reporting person may be deemed to beneficially own the shares of common stock owned by the Trust. The option to purchase 100,096 shares was granted on January 5, 2018. The shares underlying the option vest over four years, with 25% of the shares vested on January 5, 2019 and the remaining shares vesting in equal quarterly installments thereafter. /s/ Jennifer Elliott, Attorney-in-Fact 2020-08-18