Company: DAWN
Filing Date: 2025-11-13
Form Type: SC TO-C
Source: 0001104659-25-110815
Chunk: 12

Company: Day One Biopharmaceuticals, Inc.
Filing Date: 2025-11-13
Form: SC TO-C
Chunk 12
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*Certain
annexes, exhibits and schedules have been omitted pursuant to Item 601(a)(5) of Regulation S-K. The registrant hereby undertakes to furnish
supplemental copies of any of the omitted annexes and schedules upon request by the SEC.

<div align='center'>SIGNATURES</div>

Pursuant to the requirements of the Securities Exchange Act of 1934,
as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

| Date: November 13, 2025 | By: | DAY ONE BIOPHARMACEUTICALS, INC.                    
 /s/ Charles N. York II, M.B.A.                      |
|:------------------------|:----|:----------------------------------------------------|
|                         |     | Charles N. York II, M.B.A.                          |
|                         |     | Chief Operating Officer and Chief Financial Officer |

Exhibit 99.1

<div align='center'>Day One to Acquire Mersana Therapeutics to Advance Mission to Bring New Medicines to People of All Ages with Life-Threatening Diseases

Acquisition expands Day One’s portfolio with novel antibody-drug conjugate (ADC) emiltatug ledadotin (Emi-Le)

Emi-Le has demonstrated early anti-tumor activity in an ongoing Phase 1 study for adenoid cystic carcinoma type-1 (ACC-1), a cancer with high unmet need and a lack of therapeutic options

Day One will apply existing scientific and commercial capabilities to this investigational innovative therapeutic with a clear potential registration path

Company to host conference call and webcast today, November 13, 8:00 am Eastern Time</div>

BRISBANE, Calif., Nov. 13, 2025 – Day One Biopharmaceuticals, Inc. (Nasdaq: DAWN) (“Day One” or the “Company”),
a biopharmaceutical company dedicated to developing and commercializing targeted therapies for people of all ages with life-threatening
diseases, today announced it has signed a definitive merger agreement for Day One to acquire Mersana Therapeutics, Inc. (“Mersana”).

The transaction positions the Company for continued success and expansion
of its oncology portfolio, adding Emi-Le, a potential first-in-class monotherapy for patients with ACC, an aggressive