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X0202 SCHEDULE 13D/A 0001863302 XXXXXXXX LIVE 5 Common Stock, par value $0.0001 per share 04/09/2026 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 12929322.00 0.00 12929322.00 0.00 12929322.00 N 12.52 OO (1) All percentages of ownership of Common Stock by the Reporting Persons presented in this Statement are based on 103,297,691 shares of Common Stock outstanding as of February 19, 2026, as reported in the Issuer's Annual Report on Form 10-K for the year ended December 31, 2025, filed with the Securities and Exchange Commission (the "SEC") on February 24, 2026. 0001391297 N Access Industries Holdings LLC AF N DE 0.00 12929322.00 0.00 12929322.00 12929322.00 N 12.52 OO (1) All percentages of ownership of Common Stock by the Reporting Persons presented in this Statement are based on 103,297,691 shares of Common Stock outstanding as of February 19, 2026, as reported in the Issuer's Annual Report on Form 10-K for the year ended December 31, 2025, filed with the SEC on February 24, 2026. 0001508226 N Access Industries Management, LLC AF N DE 0.00 12929322.00 0.00 12929322.00 12929322.00 N 12.52 OO (1) All percentages of ownership of Common Stock by the Reporting Persons presented in this Statement are based on 103,297,691 shares of Common Stock outstanding as of February 19, 2026, as reported in the Issuer's Annual Report on Form 10-K for the year ended December 31, 2025, filed with the SEC on February 24, 2026. 0001326628 N Len Blavatnik AF N X1 0.00 12929322.00 0.00 12929322.00 12929322.00 N 12.52 IN (1) All percentages of ownership of Common Stock by the Reporting Persons presented in this Statement are based on 103,297,691 shares of Common Stock outstanding as of February 19, 2026, as reported in the Issuer's Annual Report on Form 10-K for the year ended December 31, 2025, filed with the SEC on February 24, 2026. Common Stock, par value $0.0001 per share Day One Biopharmaceuticals, Inc. 1800 Sierra Point Parkway Suite 200 Brisbane CA 94005 This Amendment No. 5 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 and Amendment No. 4 to the Schedule 13D filed by the Reporting Persons with the SEC on August 1, 2024 (together, the "Schedule") is hereby amended and supplemented by the Reporting Persons as set forth below in this Amendment No. 5. 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 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. 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. Item 5 of the Schedule 13D is hereby amended and restated as follows: The responses of each of the Reporting Persons with respect to Rows 11, 12, and 13 of the cover pages of this Schedule 13D that relate to the aggregate number and percentage of Common Stock (including but not limited to footnotes to such information) are incorporated herein by reference. The responses of each of the Reporting Persons with respect to Rows 7, 8, 9, and 10 of the cover pages of this Schedule 13D that relate to the number of Common Stock as to which each of the persons or entities referenced in Item 2 above has sole or shared power to vote or to direct the vote of and sole or shared power to dispose of or to direct the disposition of (including but not limited to footnotes to such information) are incorporated herein by reference. 12,929,322 shares of Common Stock and Warrants to purchase 827,586 shares of Common Stock are owned directly by AI Day1 and may be deemed to be beneficially owned by AIH, AIM and Mr. Blavatnik because (i) AIH indirectly controls all of the outstanding voting interests in AI Day1, (ii) AIM controls AIH and (iii) Mr. Blavatnik controls AIM and controls a majority of the outstanding voting interests in AIH. Each of the Reporting Persons (other than AI Day1), and each of their affiliated entities and the officers, partners, members and managers thereof, disclaims beneficial ownership of these securities. The following transactions in the Issuer's securities have been effected by Reporting Persons within the 60 days prior to this filing: The information set forth in Item 3 of this Schedule 13D is incorporated by reference herein. Not applicable. Not applicable. AI Day1 LLC /s/ Alejandro Moreno By: Access Industries Management, LLC, its Manager; its Executive Vice President/Alejandro Moreno 04/13/2026 Access Industries Holdings LLC /s/ Alejandro Moreno By: Access Industries Management, LLC, its Manager; its Executive Vice President/Alejandro Moreno 04/13/2026 Access Industries Management, LLC /s/ Alejandro Moreno By: Executive Vice President/Alejandro Moreno 04/13/2026 Len Blavatnik * Len Blavatnik 04/13/2026 * The undersigned, by signing his name hereto, executes this Amendment No. 5 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