X0306 4 2020-06-16 1 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 2020-06-16 4 C 0 113231 A 1479080 D Common Stock 226462 I See Footnote Series A Preferred Stock 2020-06-16 4 C 0 200000 0.00 D Common Stock 113321 0 D On June 16, 2020, the Series A Preferred Stock automatically converted into Common Stock on a 1.7663-for-one basis without payment of further consideration upon closing of the initial public offering of the Issuer's common stock. The shares had no expiration date. 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. President and Chief Executive Officer /s/ Jennifer Elliott, Attorney-in-Fact 2020-06-18