Form 4 Filing by David Tsao

This Form 4 filing details transactions made by David Tsao, Chief Technology Officer and Director of BillionToOne, Inc. On September 24, 2026, Tsao acquired 3,333 shares of Class A Common Stock at a price of $2.80 per share under a Rule 10b5-1 trading plan. Concurrently, Tsao disposed of 3,333 shares of Class A Common Stock at a price of $125.00 per share. Additionally, Tsao exercised a stock option to acquire 3,333 shares of Class A Common Stock at an exercise price of $2.80 per share. Following these transactions, Tsao directly owns 1,000 shares of Class A Common Stock and has options for 556,667 shares.