X0206 3 2020-06-11 0 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 Series B Preferred Stock Common Stock 703331 I See Footnote Series C Preferred Stock Common Stock 257883 I See Footnote The Series B 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 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. The Series C 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. Each Reporting Person disclaims beneficial ownership in the securities reported on this Form 3 except to the extent of its or his pecuniary interest, if any, therein, and this report shall not be deemed to be an admission that such Reporting Person is the beneficial owner of such securities for purposes of Section 16 or for any other purpose. /s/ Casdin Capital, LLC, By: /s/ Eli Casdin, Managing Member 2020-06-11 /s/ Eli Casdin 2020-06-11