X0508 4 2023-09-14 0 0001816736 Disc Medicine, Inc. IRON 0001628098 Atlas Venture Fund X, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001742766 ATLAS VENTURE ASSOCIATES X, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001742765 Atlas Venture Associates X, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0 Common Stock 2023-09-14 4 S 0 94440 53.05 D 1856794 D Common Stock 2023-09-15 4 S 0 260894 53.16 D 1595900 D Common Stock 2023-09-16 4 S 0 2115 53.22 D 1593785 D The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $53.00 to $53.42 inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in footnotes (1), (3) and (4). These shares are held directly by Atlas Venture Fund X, L.P. ("Atlas X"). Atlas Venture Associates X, L.P. ("Associates X") is the general partner of Atlas X. Atlas Venture Associates X, LLC ("Associates X LLC") is the general partner of Associates X. Each of Associates X and Associates X LLC disclaims Section 16 beneficial ownership of the securities held by Atlas X, except to the extent of its pecuniary interest therein, if any. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $53.00 to $53.60 inclusive. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $53.05 to $53.365 inclusive. Atlas Venture Fund X, L.P., By: Atlas Venture Associates X, L.P., its general partner, By: Atlas Venture Associates X, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 2023-09-18 Atlas Venture Associates X, L.P., By: Atlas Venture Associates X, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 2023-09-18 Atlas Venture Associates X, LLC By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 2023-09-18