---
schema_version: "secwatch.filing_event.v1"
accession: "0001104659-26-001127"
form_type: "8-K"
ticker: null
cik: "0001845337"
company_name: "Day One Biopharmaceuticals, Inc."
filed_at: "2026-01-06T23:59:59+00:00"
generated_at: "2026-05-16T11:33:29.464625+00:00"
event_type: "m_and_a"
sentiment: "neutral"
materiality_score: 0.9
calibrated_materiality_score: 0.9
confidence: "high"
source: SEC EDGAR
---

# Day One completes acquisition of Mersana for $25/share cash plus CVRs up to $30.25

## Summary
- Acquisition closed Jan 6, 2026; Mersana becomes wholly owned subsidiary of Day One.
- Tender offer saw 60.57% of Mersana shares tendered; total cash consideration ~$128.8M.
- Each Mersana share received $25 cash plus one non-tradable CVR with potential milestone payments up to $30.25/share.
- CVR milestones include FDA breakthrough designation and sales thresholds for emiltatug-ledadotin (Emi-Le).
- Mersana common stock to be delisted from Nasdaq effective Jan 6, 2026.

## SEC filing metadata
- accession: 0001104659-26-001127
- form_type: 8-K
- cik: 0001845337
- company_name: Day One Biopharmaceuticals, Inc.
- filed_at: 2026-01-06T23:59:59+00:00
- event_type: m_and_a
- sentiment: neutral
- materiality_score: 0.9
- calibrated_materiality_score: 0.9
- confidence: high
- sec_items: 1.01, 2.01, 7.01, 9.01
- EDGAR index: https://www.sec.gov/Archives/edgar/data/1845337/000110465926001127/0001104659-26-001127-index.htm
- EDGAR primary document: https://www.sec.gov/Archives/edgar/data/1845337/000110465926001127/tm261757d1_8k.htm

## Machine-readable alternates
- HTML: https://secwatch.observer/filing/0001104659-26-001127
- JSON: https://secwatch.observer/filing/0001104659-26-001127.json
- Plain text: https://secwatch.observer/filing/0001104659-26-001127.txt

## Source-grounded claims
- claim_id: ae0dfac7e018bf04400b916e549bcc922e514059
  claim: Day One Biopharmaceuticals, Inc. completed an acquisition involving Mersana Therapeutics, Inc. for $25.00 per Target Share plus one non-tradable contingent value right (closed 2026-01-06).
  evidence_excerpt: offer to purchase all of the issued and outstanding shares (the “ Target Shares ”) of Target’s common stock, par value $0.0001 per share (the “ Target Common Stock ”), for (i) $25.00 per Target Share (the “ Upfront Consideration ”), net to the stockholder in cash, without interest and less any applicable tax withholding, plus (ii) one non-tradable contingent
  evidence_url: https://www.sec.gov/Archives/edgar/data/1845337/000110465926001127/0001104659-26-001127-index.htm
- claim_id: 82a20b74b5cbdefc730e1ca4862fc81c9a08b8d5
  claim: Day One Biopharmaceuticals, Inc. entered into CVR Agreement with Rights Agent (effective 2026-01-06).
  evidence_excerpt: on January 6, 2026, the Parent and the Rights Agent entered into the CVR Agreement governing the terms of the CVRs issued pursuant to the Offer and the Merger
  evidence_url: https://www.sec.gov/Archives/edgar/data/1845337/000110465926001127/0001104659-26-001127-index.htm

This AI-assisted summary is a reading aid. Review the linked SEC EDGAR filing before relying on any specific claim.