X0202 SCHEDULE 13D/A 0001863302 XXXXXXXX LIVE 6 Common Stock, par value $0.0001 per share 04/23/2026 false 0001845337 23954D109 Day One Biopharmaceuticals, Inc.

1800 Sierra Point Parkway Suite 200 Brisbane CA 94005

Alejandro Moreno (212) 247-6400 c/o Access Industries, Inc. 40 West 57th Street, 28th Floor New York NY 10019 Langhorne S. Perrow (212) 247-6400 c/o Access Industries, Inc. 40 West 57th Street, 28th Floor New York NY 10019 Nicholas P. Pellicani 44 20 7786 9000 Debevoise & Plimpton LLP The Northcliffe, 28 Tudor St. London X0 EC4Y 0AY 0001863302 N AI Day1 LLC AF N DE 0.00 0.00 0.00 0.00 0.00 N 0.00 OO 0001391297 N Access Industries Holdings LLC AF N DE 0.00 0.00 0.00 0.00 0.00 N 0.00 OO 0001508226 N Access Industries Management, LLC AF N DE 0.00 0.00 0.00 0.00 0.00 N 0.00 OO 0001326628 N Len Blavatnik AF N X1 0.00 0.00 0.00 0.00 0.00 N 0.00 IN Common Stock, par value $0.0001 per share Day One Biopharmaceuticals, Inc. 1800 Sierra Point Parkway Suite 200 Brisbane CA 94005 This Amendment No. 6 to Schedule 13D is being filed by AI Day1 LLC ("AI Day1"), Access Industries Holdings LLC ("AIH"), Access Industries Management, LLC ("AIM") and Len Blavatnik (collectively, the "Reporting Persons", and each, a "Reporting Person"), in respect of the common stock, par value $0.0001 per share (the "Common Stock"), of Day One Biopharmaceuticals, Inc. (the "Issuer"). The Schedule 13D filed with the Securities and Exchange Commission ("SEC") on June 9, 2021, as amended and supplemented by Amendment No. 1 to the Schedule 13D filed by the Reporting Persons with the SEC on June 21, 2022, Amendment No. 2 to the Schedule 13D filed by the Reporting Persons with the SEC on June 12, 2023, Amendment No. 3 to the Schedule 13D filed by the Reporting Persons with the SEC on October 20, 2023, Amendment No. 4 to the Schedule 13D filed by the Reporting Persons with the SEC on August 1, 2024 and Amendment No. 5 to the Schedule 13D filed by the Reporting Persons with the SEC on April 13, 2026 (together, the "Schedule") is hereby amended and supplemented by the Reporting Persons as set forth below in this Amendment No. 6. This amendment is filed by the Reporting Persons in accordance with Rule 13d-2 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), and refers only to information that has materially changed since the filing of the Schedule. The items identified below, or the particular paragraphs of such items which are identified below, are amended as set forth below. Unless otherwise indicated, all capitalized terms used and not defined herein have the respective meanings assigned to them in the Schedule. The disclosure in Item 4 is hereby amended and supplemented by adding the following at the end thereof: On April 9, 2026, AI Day1 tendered all of its shares of Common Stock pursuant to the tender offer by Servier Detroit Inc. ("Purchaser") to purchase all of the issued and outstanding shares of Common Stock for $21.50 per share in cash, upon the terms and subject to the conditions described in the Offer to Purchase and related Letter of Transmittal filed with the SEC on Schedule TO on March 26, 2026 (the "Tender Offer"). Such shares were accepted by the Purchaser on April 23, 2026. On April 23, 2026, in connection with the consummation of the merger, each pre-funded warrant held by AI Day1 to purchase Common Stock was converted into the right to receive $21.4999 per warrant in cash. Item 5 of the Schedule 13D is hereby amended and restated as follows: As a result of the Tender Offer, the Reporting Persons no longer have any beneficial ownership in the Common Stock. The information set forth in Item 4 of this Schedule 13D is incorporated by reference herein. Not applicable. The Reporting Persons ceased to be beneficial owners of more than five percent of the Issuer's Common Stock on April 23, 2026. AI Day1 LLC /s/ Alejandro Moreno 04/27/2026 Access Industries Holdings LLC /s/ Alejandro Moreno 04/27/2026 Access Industries Management, LLC /s/ Alejandro Moreno 04/27/2026 Len Blavatnik * 04/27/2026 * The undersigned, by signing his name hereto, executes this Amendment No. 6 to Schedule 13D pursuant to the Limited Power of Attorney executed on behalf of Mr. Blavatnik and filed herewith. By: /s/ Alejandro Moreno Name: Alejandro Moreno Attorney-in-Fact