SCHEDULE 13D/A

Foghorn Therapeutics Inc

Common Stock, $0.0001 par value per share · CUSIP: 344174107

Event date: 2026-01-09

Reporting persons may disclose overlapping interests. Their amounts are shown individually and are not added into a filing total.

Filing-level shares

Unavailable

Percent of class

Unavailable

• Flagship Ventures Fund V, L.P.Reported CIK 0001627639

  Shares: 9,330,878 · Class: 15.9%

• Flagship Ventures Fund V General Partner LLC

  Shares: 9,330,878 · Class: 15.9%

• Flagship Ventures Opportunities Fund I, L.P.

  Shares: 1,491,441 · Class: 2.5%

• Flagship Ventures Opportunities Fund I General Partner LLC

  Shares: 1,491,441 · Class: 2.5%

• Flagship Pioneering Special Opportunities Fund II, L.P.

  Shares: 3,666,715 · Class: 3.2%

• Flagship Pioneering Special Opportunities Fund II General Partner LLC

  Shares: 3,666,715 · Class: 3.2%

• Flagship Pioneering Fund VII, L.P.

  Shares: 6,285,851 · Class: 7.1%

• Flagship Pioneering Fund VII General Partner LLC

  Shares: 6,285,851 · Class: 7.1%

• Flagship Pioneering, LLC

  Shares: 9,952,566 · Class: 10.0%

• Noubar Afeyan

  Shares: 20,774,885 · Class: 27.2%

Amounts are as disclosed in this filing. Schedule type alone does not establish investment intent.