X0306 4 2022-11-01 0 0001845337 Day One Biopharmaceuticals, Inc. DAWN 0001863107 Bender Jeremy 2000 SIERRA POINT PARKWAY, SUITE 501 BRISBANE CA 94080 1 1 0 0 Chief Executive Officer Common Stock 2022-11-01 4 S 0 12500 20.0284 D 1215375 D Common Stock 281574 I See Footnote Common Stock 281574 I See Footnote The sales reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $20.00 to $20.55 per share, 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 The Jeremy Bender 2022 Grantor Retained Annuity Trust under Irrevocable Trust Agreement dated March 29, 2022 of which the Reporting Person is trustee. Represents shares held by The Melissa Bender 2022 Grantor Retained Annuity Trust under Irrevocable Trust Agreement dated March 29, 2022 of which the Reporting Person's spouse is trustee. /s/ Charles N. York II, Attorney-in-Fact 2022-11-03