X0306 4 2022-09-16 0 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 2022-09-16 4 S 0 111585 21.53 D 9307021 D Common Stock 2022-09-16 4 S 0 3722 22.04 D 9303299 D Common Stock 2022-09-19 4 S 0 76725 20.57 D 9226574 D Common Stock 2022-09-20 4 S 0 146404 19.50 D 9080170 D Common Stock 2022-09-20 4 S 0 2266 20.10 D 9077904 D The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $20.98 - $21.97, 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 $21.98 - $22.28, 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. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $20.22 - $21.07, 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. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $19.06 - $19.85, 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. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $20.06 - $20.15, 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. Canaan XI L.P., By: Canaan Partners XI LLC, its general partner, By: /s/ Nancy Levenson, Attorney-in-Fact 2022-09-20 Canaan Partners XI LLC, By: /s/ Nancy Levenson, Attorney-in-Fact 2022-09-20