X0306 4/A 2022-07-01 2022-07-06 0 0001845337 Day One Biopharmaceuticals, Inc. DAWN 0001673237 Grant Papanek Julie 395 OYSTER POINT BLVD, SUITE 217 SOUTH SAN FRANCISCO CA 94080 1 0 0 0 Common Stock 2022-07-01 4 S 0 1000 20.00 D 464000 I See Footnote Common Stock 2022-07-05 4 S 0 800 20.00 D 463200 I See Footnote Common Stock 362 I See Footnote Common Stock 9663645 I See Footnote The sales reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan previously adopted by the Reporting Person on September 28, 2021. 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 Securities Exchange Act of 1934, as amended (the "Act"), or for any other purpose. This amendment to Form 4 is filed to include holdings by The Adam and Julie Grant Revocable Trust dtd October 13, 2017 (the "Revocable Trust"), of which the Reporting Person and her spouse are co-trustees, that were inadvertently omitted from the Form 4 filed on July 6, 2022. These shares were received by the Revocable Trust in one or more prior distributions-in-kind by Canaan XI L.P. for no consideration pursuant to the exemptions afforded by Rules 16a-9 and 16a-13 of the Act. 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. 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 Act, or for any other purpose. /s/ Charles N. York II, Attorney-in-Fact 2022-07-12