X0306 4 2020-06-16 0 0001733294 Generation Bio Co. GBIO 0001577014 Rhodes Jason P C/O GENERATION BIO CO., 301 BINNEY STREET CAMBRIDGE MA 02142 1 0 0 0 Common Stock 2020-06-16 4 C 0 10040032 A 12630758 I See Footnote Series A Preferred Stock 2020-06-16 4 C 0 15051020 0.00 D Common Stock 8521213 0 I See Footnote Series C Preferred Stock 2020-06-16 4 C 0 2682.69 0.00 D Common Stock 1518819 0 I See Footnote 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. 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. Includes 11,111,939 shares held by Atlas Venture Fund X, L.P. ("Atlas Venture Fund X") and 1,518,819 shares held by Atlas Venture Opportunity Fund I, L.P. ("Atlas Venture Opportunity Fund I"). The general partner of Atlas Venture Fund X is Atlas Venture Associates X, L.P. ("AVA X LP"). Atlas Venture Associates X, LLC ("AVA X LLC") is the general partner of AVA X LP. The reporting person is a member of AVA X LLC and disclaims beneficial ownership of such securities held by Atlas Venture Fund X, except to the extent of his pecuniary interest therein, if any. The general partner of Atlas Venture Opportunity Fund I is Atlas Venture Associates Opportunity I, L.P. ("AVAO I LP"). Atlas Venture Associates Opportunity I, LLC ("AVAO I LLC") is the general partner of AVAO I LP. The reporting person is a member of AVAO I LLC and disclaims beneficial ownership of such securities held by Atlas Venture Opportunity Fund I, except to the extent of his pecuniary interest therein, if any. The shares are held directly by Atlas Venture Fund X. The general partner of Atlas Venture Fund X is AVA X LP. AVA X LLC is the general partner of AVA X LP. The reporting person is a member of AVA X LLC and disclaims beneficial ownership of such securities held by Atlas Venture Fund X, except to the extent of his pecuniary interest therein, if any. The shares are held directly by Atlas Venture Opportunity Fund I. The general partner of Atlas Venture Opportunity Fund I is AVAO I LP. AVAO I LLC is the general partner of AVAO I LP. The reporting person is a member of AVAO I LLC and disclaims beneficial ownership of such securities held by Atlas Venture Opportunity Fund I, except to the extent of his pecuniary interest therein, if any. /s/ Ommer Chohan, Attorney-in-Fact 2020-06-18