X0306 4 2021-02-01 0 0001845337 Day One Biopharmaceuticals, Inc. DAWN 0001708269 Atlas Venture Fund XI, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001780066 Atlas Venture Associates Opportunity I, LLC 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 0001780065 Atlas Venture Associates Opportunity I, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001780068 Atlas Venture Associates XI, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 0001780067 Atlas Venture Associates XI, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE MA 02139 0 0 1 0 Common Stock 2021-06-01 4 C 0 7761380 A 7761380 I See Footnote Common Stock 2021-06-01 4 C 0 741396 A 741396 I See Footnote Common Stock 2021-06-01 4 P 0 250000 16.00 A 250000 I See Footnote Common Stock 2021-06-01 4 P 0 250000 16.00 A 250000 I See Footnote Series B Preferred Stock 2021-02-01 4 P 0 741396 0.00 A Common Stock 741396 741396 I See Footnote Series A Preferred Stock 2021-06-01 4 C 0 7761380 0.00 D Common Stock 7761380 0 I See Footnote Series B Preferred Stock 2021-06-01 4 C 0 741396 0.00 D Common Stock 741396 0 I See Footnote The shares of the Issuer's Series A Preferred Stock automatically converted on a one-for-one basis into shares of the Issuer's Common Stock on May 26, 2021 in connection with the closing of the Issuer's sale of its Common Stock in its firm commitment initial public offering (the "IPO") pursuant to a registration statement on Form S-1 (File No. 333-255754) under the Securities Act of 1933, as amended (the "Registration Statement"), and had no expiration date. The shares are held directly by Atlas Venture Fund XI, L.P. ("Fund XI"). The general partner of Fund XI is Atlas Venture Associates XI, L.P. ("AVA XI LP"). Atlas Venture Associates XI, LLC ("AVA XI LLC") is the general partner of AVA XI LP. Each of AVA XI LP and AVA XI LLC disclaims Section 16 beneficial ownership of the securities held by Fund XI, except to the extent of its pecuniary interest therein, if any. The shares of the Issuer's Series B Preferred Stock automatically converted on a one-for-one basis into shares of the Issuer's Common Stock on May 26, 2021 in connection with the closing of the Issuer's sale of its Common Stock in its IPO pursuant to the Registration Statement, and had no expiration date. The shares are held directly by Atlas Venture Opportunity Fund I, L.P. ("Atlas Venture Opportunity Fund I"). 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. On June 1, 2021, Fund XI purchased 250,000 shares of common stock of the Issuer at a purchase price of $16.00 per share pursuant to an underwritten public offering. On June 1, 2021, Atlas Venture Opportunity Fund I purchased 250,000 shares of common stock of the Issuer at a purchase price of $16.00 per share pursuant to an underwritten public offering. The Reporting Person is reporting the purchase of these securities pursuant to Rule 16a-2(a) of the Securities Exchange Act of 1934, as amended. Each share of Series B Preferred Stock automatically converted into one share of Common Stock at the close of the Issuer's IPO and has no expiration date. /s/ Ommer Chohan 2021-06-02