X0306 4 2020-06-16 0 0001733294 Generation Bio Co. GBIO 0001628098 Atlas Venture Fund X, L.P. 400 TECHNOLOGY SQUARE, 10TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001742765 Atlas Venture Associates X, LLC 400 TECHNOLOGY SQUARE, 10TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001742766 ATLAS VENTURE ASSOCIATES X, L.P. 400 TECHNOLOGY SQUARE, 10TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001759241 Atlas Venture Opportunity Fund I, L.P. 400 TECHNOLOGY SQUARE, 10TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001780065 Atlas Venture Associates Opportunity I, L.P. 400 TECHNOLOGY SQUARE, 10TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001780066 Atlas Venture Associates Opportunity I, LLC 400 TECHNOLOGY SQUARE, 10TH FLOOR CAMBRIDGE MA 02139 0 0 1 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 268269 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. Each of AVA X LP and AVA X LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Fund X, except to the extent of its 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. Each of AVAO I LP and AVAO I LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Opportunity Fund I, except to the extent of its 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. Each of AVA X LP and AVA X LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Fund X, except to the extent of its 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. Each of AVAO I LP and AVAO I LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Opportunity Fund I, except to the extent of its pecuniary interest therein, if any. /s/ Atlas Venture Fund X, LP, By: Atlas Venture Associates X, L.P., it's general partner, By: Atlas Venture Associates X, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 2020-06-18 /s/ Atlas Venture Associates X, LP, By: Atlas Venture Associates X, LLC, it's general partner, By: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 2020-06-18 /s/ Atlas Venture Associates X, LLC, By: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 2020-06-18 /s/ Atlas Venture Opportunity Fund I, L.P., By: Atlas Venture Associates Opportunity I, L.P., its general partner, By: Atlas Venture Associates Opportunity I, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 2020-06-18 /s/ Atlas Venture Associates Opportunity I, L.P., By: Atlas Venture Associates Opportunity I, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 2020-06-18 /s/ Atlas Venture Associates Opportunity I, LLC, By: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 2020-06-18