DAWN

• The tender offer expired on April 22, 2026, with 88,180,910 shares (85.34%) validly tendered, satisfying the minimum tender condition.
• Each outstanding share of Day One common stock was converted into the right to receive $21.50 per share in cash, totaling approximately $2.5 billion in equity value.
• As a result of the merger, Day One's stock will be delisted from Nasdaq and its registration will be terminated.
• The company's board of directors and officers were replaced by those of the purchaser, and the certificate of incorporation and bylaws were amended and restated.

1.02 Termination of a Material Definitive Agreement · 2.01 Completion of Acquisition or Disposition of Assets · 3.01 Notice of Delisting or Failure to Satisfy a Continued Listing Rule or Standard; Transfer of Listing · 3.03 Material Modification to Rights of Security Holders · 5.01 Changes in Control of Registrant · 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements · 5.03 Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year · 9.01 Financial Statements and Exhibits

• Merger Sub will commence a cash tender offer for all outstanding Day One shares at $21.50 per share, net to seller in cash, subject to withholding taxes.
• The transaction represents a total equity value of approximately $2.5 billion.
• The offer is expected to close in the second half of 2026, subject to customary conditions including tender of a majority of shares and HSR waiting period expiration.
• Day One's board approved the merger and recommends stockholders tender their shares; the merger will be effected under Section 251(h) of the DGCL without a stockholder meeting.

1.01 Entry into a Material Definitive Agreement · 8.01 Other Events · 9.01 Financial Statements and Exhibits

• The company entered into an exclusive license agreement with Ipsen in July 2024 to commercialize OJEMDA outside the United States.
• The updated corporate presentation was attached as Exhibit 99.1 to the Form 8-K.
• The information was furnished under Item 7.01 Regulation FD Disclosure and is not deemed filed for SEC purposes.

7.01 Regulation FD Disclosure · 9.01 Financial Statements and Exhibits

• Full-year 2025 net loss was $107.3 million, compared to $95.5 million in 2024.
• Cash, cash equivalents, and short-term investments totaled $441.1 million as of December 31, 2025.
• Company reaffirmed 2026 U.S. OJEMDA net product revenue guidance of $225 million to $250 million.
• Pipeline includes Emi-Le (acquired via Mersana Therapeutics) in Phase 1 for adenoid cystic carcinoma and DAY301 in Phase 1a dose escalation.

2.02 Results of Operations and Financial Condition · 7.01 Regulation FD Disclosure · 9.01 Financial Statements and Exhibits

• Preliminary unaudited OJEMDA net product revenue was approximately $52.8 million for Q4 2025 and $155.4 million for full year 2025.
• Company projects 2026 U.S. OJEMDA net product revenue of $225–$250 million, representing 53% growth at the midpoint.
• As of December 31, 2025, cash, cash equivalents, and short-term investments were approximately $441.1 million (unaudited, pre-Mersana acquisition).
• 2026 milestones include completing FIREFLY-2 enrollment in H1 2026, initial DAY301 Phase 1a data in H2 2026, and Emi-Le Phase 1 data by mid-2026.

2.02 Results of Operations and Financial Condition · 7.01 Regulation FD Disclosure · 9.01 Financial Statements and Exhibits

• On January 6, 2026, Day One Biopharmaceuticals completed its acquisition of Mersana Therapeutics through a merger of its subsidiary, Emerald Merger Sub, into Mersana.
• The CVRs provide for potential milestone payments totaling up to $30.25 per CVR, tied to regulatory and commercial milestones for emiltatug ledadotin (Emi-Le), including FDA breakthrough therapy designation and sales thresholds.
• The tender offer expired on January 5, 2026, with approximately 60.57% of Mersana's outstanding shares validly tendered, satisfying the minimum condition.
• Day One stated the acquisition expands its pipeline with the clinical-stage antibody drug conjugate Emi-Le, targeting adenoid cystic carcinoma (ACC).

1.01 Entry into a Material Definitive Agreement · 2.01 Completion of Acquisition or Disposition of Assets · 7.01 Regulation FD Disclosure · 9.01 Financial Statements and Exhibits

• In 76 evaluable patients, overall response rate was 53% (40/76), median duration of response was 19.4 months, and median progression-free survival was 16.6 months.
• Median time-to-next-treatment was 42.6 months; 77% of patients entering a treatment-free observation period remained off therapy for at least 12 months.
• Eight retreated patients had a median retreatment duration of 9 months and a median maximum tumor reduction of -38.3%.
• No new safety signals were identified; common grade 3+ adverse events included decreased growth velocity, anemia, and increased creatine phosphokinase.

7.01 Regulation FD Disclosure · 8.01 Other Events · 9.01 Financial Statements and Exhibits

• Stockholders will receive $25.00 per share in cash plus one contingent value right (CVR) per share, with potential milestone payments up to $30.25 per share.
• The merger is expected to close by the end of January 2026, subject to customary conditions including tender of a majority of shares and HSR clearance.
• Mersana's board unanimously recommends that stockholders tender their shares in the offer.
• Support stockholders, including directors, executive officers, and Bain Capital affiliates, holding about 8.5% of shares, have agreed to tender their shares.

1.01 Entry into a Material Definitive Agreement · 7.01 Regulation FD Disclosure · 9.01 Financial Statements and Exhibits