Amendment to Schedule 13D for Sionna Therapeutics, Inc.

This Amendment No. 1 to Schedule 13D, filed by Atlas Venture Fund XI, L.P. and related entities, updates their ownership in Sionna Therapeutics, Inc. The reporting entities collectively own 7.0% of the company's outstanding common stock, with Atlas Venture Fund XI, L.P. holding 5.4% and Atlas Venture Opportunity Fund II, L.P. holding 1.6%. The amendment details recent transactions involving the sale of shares between September 29, 2025, and October 20, 2025, with prices ranging from $29.95 to $33.86 per share. The reporting persons disclaim membership in a group for purposes of Section 13 of the Securities Exchange Act of 1934.