X0306 4 2021-06-16 0 0001733294 Generation Bio Co. GBIO 0001628098 Atlas Venture Fund X, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001759241 Atlas Venture Opportunity Fund I, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 Common Stock 2021-06-16 4 J 0 1000000 0.00 D 8111939 D Common Stock 2021-06-16 4 J 0 100000 0.00 D 1266937 I See footnote Represents a pro rata distribution, and not a purchase or sale of securities, by Atlas Venture Fund X, L.P. ("Atlas Venture Fund X") to its general partner and limited partners without additional consideration. This distribution was effected pursuant to a plan adopted by Atlas Venture Fund X pursuant to Rule 10b5-1 on April 20, 2021. Represents a pro rata distribution, and not a purchase or sale of securities, by Atlas Venture Opportunity Fund I, LP ("AVOF I") to its general partner and limited partners without additional consideration. This distribution was effected pursuant to a plan adopted by AVOF I pursuant to Rule 10b5-1 on April 20, 2021. The shares are owned directly by AVOF I. Atlas Venture Fund X, LP, By: Atlas Venture Associates X, L.P., its general partner, By: Atlas Venture Associates X, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer 2021-06-21 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 2021-06-21