X0306 4 2022-09-21 0 0001845337 Day One Biopharmaceuticals, Inc. DAWN 0001673237 Grant Julie Papanek 395 OYSTER POINT BLVD, SUITE 217 SOUTH SAN FRANCISCO CA 94080 1 0 0 0 Common Stock 2022-09-21 4 S 0 607497 18.54 D 8470407 I See Footnote Common Stock 2022-09-21 4 S 0 6106 19.29 D 8464301 I See Footnote Common Stock 2022-09-21 4 J 0 1500000 D 6964301 I See Footnote Common Stock 439358 I See Footnote Common Stock 868 I See Footnote 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 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 range set forth in this footnote. Represents shares held by Canaan XI L.P. The Reporting Person is a non-managing member of Canaan Partners XI LLC, the general partner of Canaan XI L.P. The Reporting Person does not have voting or dispositive power over any of the shares held directly by Canaan XI L.P. and disclaims beneficial ownership of the shares except to the extent of her pecuniary interest therein, if any, and this report shall not be deemed an admission that the Reporting Person is the beneficial owner of the shares for purposes of Section 16 of the Securities Exchange Act of 1934, as amended (the "Act"), or for any other purpose. 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 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 range set forth in this footnote. On September 21, 2022, Canaan XI L.P. distributed, for no consideration, 1,500,000 shares of common stock of the issuer to its limited partners and to Canaan Partners XI LLC, the general partner of Canaan XI L.P., representing each such partner's pro rata interest in the shares held by Canaan XI L.P. On the same date, Canaan Partners XI LLC 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 Act. Represents shares held by The Grant Family Delaware Trust Agreement of which the Reporting Person is trustee. The Reporting Person disclaims beneficial ownership of the shares except to the extent of her pecuniary interest therein, if any, and this report shall not be deemed an admission that the Reporting Person is the beneficial owner of the shares for purposes of Section 16 of the Act, or for any other purpose. Includes shares received in the distributions described in footnote (4) above. Represents shares held by The Adam and Julie Grant Revocable Trust dtd October 13, 2017 of which the Reporting Person and her spouse are co-trustees. The Reporting Person disclaims beneficial ownership of the shares except to the extent of her pecuniary interest therein, if any, and this report shall not be deemed an admission that the Reporting Person is the beneficial owner of the shares for purposes of Section 16 of the Act, or for any other purpose. /s/ Nancy Levenson, Attorney-in-Fact 2022-09-22