X0306 4 2020-06-16 1 0001733294 Generation Bio Co. GBIO 0001534261 Casdin Capital, LLC 1350 AVENUE OF THE AMERICAS SUITE 2600 NEW YORK NY 10019 0 0 1 0 0001534264 Casdin Eli 1350 AVENUE OF THE AMERICAS SUITE 2600 NEW YORK NY 10019 0 0 1 0 Common Stock 2020-06-16 4 C 0 961215 A 961215 I See Footnote Common Stock 2020-06-16 4 P 0 650000 19.00 A 1611215 I See Footnote Series B Preferred Stock 2020-06-16 4 C 0 1093410 0.00 D Common Stock 703331 0 I See Footnote Series C Preferred Stock 2020-06-16 4 C 0 455500 0.00 D Common Stock 257883 0 I See Footnote On June 16, 2020, the Series B Preferred Stock automatically converted into Common Stock on approximately a 1.5546-for-one basis (when rounded to the nearest ten-thousandth) without payment of further consideration upon closing of the initial public offering of the Issuer's common stock. The shares had no expiration date. On June 16, 2020, the Series C 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 owned directly by clients of Casdin Capital, LLC and may be deemed to be indirectly beneficially owned by (i) Casdin Capital, LLC as investment adviser to such clients and (ii) Eli Casdin as managing member of Casdin Capital, LLC. /s/ Casdin Capital, LLC, By: /s/ Eli Casdin, Managing Member 2020-06-18 /s/ Eli Casdin 2020-06-18