Company: DAWN
Filing Date: 2025-12-05
Form Type: SC TO-T
Source: 0001104659-25-118858
Chunk: 1

Company: Day One Biopharmaceuticals, Inc.
Filing Date: 2025-12-05
Form: SC TO-T
Chunk 1
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4(i) (Cross-Border Issuer Tender Offer)

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Rule 14d-1(d) (Cross-Border Third-Party Tender Offer)

Items 1 through 9 and Item 11. This Tender Offer Statement on Schedule TO (together with any amendments and supplements hereto, the “Schedule TO”) relates to the offer by Emerald Merger Sub, Inc., a Delaware corporation and a direct wholly owned subsidiary of Day One Biopharmaceuticals, Inc., a Delaware corporation (“Day One”), to purchase all of the issued and outstanding shares of common stock, par value $0.0001 per share (the “Shares”), of Mersana Therapeutics, Inc., a Delaware corporation (“Mersana”), which constitute all of the issued and outstanding shares of capital stock of Mersana, at a purchase price of (i) $25.00 per Share, net to the stockholder in cash, without interest and less any applicable tax withholding, plus (ii) one non-tradable contingent value right (each a “CVR”) per Share, representing the right to receive certain contingent payments of up to an aggregate amount of $30.25 per CVR in cash, without interest, less any applicable withholding taxes, upon the achievement of certain specified milestones on or prior to the applicable milestone outside dates in accordance with the terms and conditions set forth in the Contingent Value Rights Agreement, to be entered into with a rights agent mutually agreeable to Day One and Mersana, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated December 5, 2025 (as it may be amended or supplemented from time to time, the “Offer to Purchase”), and in the related Letter of Transmittal (as it may be amended or supplemented from time to time, the “Letter of Transmittal”), copies of which are attached hereto as Exhibits (a)(1)(A) and (a)(1)(B), respectively. All information contained in the Offer to Purchase (including Schedule I thereto) and the related Letter of Transmittal is hereby expressly incorporated herein by reference in response to Items 1 through 9 and Item 11 of this Schedule TO, except as otherwise set forth below. Item 10. Financial Statements. Not applicable. Item 12. Exhibits.

| ​ | Exhibit No. | ​ | ​ | Description                                                                                                                                                                                                                                                                                  | ​ |
| ​ | (a)(1)(A)*  | ​ |