X0508 4 2023-11-15 0 0001845337 Day One Biopharmaceuticals, Inc. DAWN 0001863107 Bender Jeremy 2000 SIERRA POINT PARKWAY, SUITE 501 BRISBANE CA 94005 1 1 0 0 Chief Executive Officer 0 Common Stock 2023-11-15 4 M 0 3562 A 707786 D Common Stock 2023-11-15 4 M 0 4750 A 712536 D Common Stock 2023-11-16 4 S 0 3107 11.69 D 709429 D Common Stock 221837 I See Footnote Common Stock 300025 I See Footnote Common Stock 251636 I See Footnote Common Stock 270226 I See Footnote Restricted Stock Units 2023-11-15 4 M 0 3562 0.00 D Common Stock 3562 28504 D Restricted Stock Units 2023-11-15 4 M 0 4750 0.00 D Common Stock 4750 57000 D Each Restricted Stock Unit ("RSU") represents a contingent right to receive one share of the Issuer's Common Stock upon settlement for no consideration. The sale of shares is for the sole purpose of covering the Reporting Person's tax liability with respect to the settlement of RSUs. 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 Jeremy Bender 2023 Grantor Retained Annuity Trust dated June 27, 2023. 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. Represents shares held by The Melissa Bender 2023 Grantor Retained Annuity Trust, dated June 27, 2023. The RSUs vest as to 1/16th of the total award in quarterly installments on February 15, May 15, August 15 and November 15, subject to the Reporting Person's provision of service to the Issuer on each vesting date. /s/ Charles N. York II, as Attorney-in-Fact 2023-11-17