X0306 4 2022-09-21 1 0001845337 Day One Biopharmaceuticals, Inc. DAWN 0001712417 Canaan XI L.P. 285 RIVERSIDE AVENUE, SUITE 250 WESTPORT CT 06880 0 0 1 0 0001850694 Canaan Partners XI LLC 285 RIVERSIDE AVENUE, SUITE 250 WESTPORT CT 06880 0 0 1 0 Common Stock 2022-09-21 4 S 0 607497 18.54 D 8470407 D Common Stock 2022-09-21 4 S 0 6106 19.29 D 8464301 D Common Stock 2022-09-21 4 J 0 1500000 D 6964301 D The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $18.27 - $19.26, inclusive. The Reporting Persons undertake 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 range set forth in this footnote. These shares are held directly by Canaan XI L.P. (the "Canaan Fund"). Canaan Partners XI LLC ("Canaan XI") is the sole general partner of the Canaan Fund and may be deemed to have sole voting, investment and dispositive power with respect to the shares held by the Canaan Fund. Canaan XI disclaims Section 16 beneficial ownership of the securities held by the Canaan Fund, 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 $19.27 - $19.35, inclusive. The Reporting Persons undertake 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 range set forth in this footnote. On September 21, 2022, the Canaan Fund distributed, for no consideration, 1,500,000 shares of common stock of the issuer to its limited partners and to Canaan XI, the general partner of the Canaan Fund, representing each such partner's pro rata interest in the shares held by the Canaan Fund. On the same date, Canaan XI distributed, for no consideration, the shares it received from the distribution to its members in an amount equal to each such member's pro rata interest in the shares. The aforementioned distributions were made in accordance with the exemptions afforded by Rule 16a-13 and Rule 16a-9 of the Securities Exchange Act of 1934, as amended. Canaan XI L.P., By: Canaan Partners XI LLC, its general partner, By: /s/ Nancy Levenson, Attorney-in-Fact 2022-09-22 Canaan Partners XI LLC, By: /s/ Nancy Levenson, Attorney-in-Fact 2022-09-22