BillionToOne, Inc.

Disclosure of Trading Arrangements

During the fiscal quarter ended March 31, 2026, none of our directors or officers (as defined in Rule 16a-1(f) under the Exchange Act)adopted orterminated a “Rule 10b5-1 trading arrangement” or a “non-Rule 10b5-1 trading arrangement,” as each term is defined in Item 408 of Regulation S-K, except as described in the paragraphs below.

On March 6, 2026, John ten Bosch, our Senior Vice President of Laboratory Operations, entered into a pre-arranged stock trading plan. Such trading plan is intended to satisfy the affirmative defense of Rule 10b5-1(c) under the Exchange Act. Mr. ten Bosch’s plan provides for, subject to certain market price limits and other terms set forth in the plan, the sale of up to 12,750 shares of Class A common stock between June 15, 2026 and August 28, 2026.

On March 6, 2026, Nancy Johnson, our Senior Vice President of Sales and Commercial Operations, entered into a pre-arranged stock trading plan. Such trading plan is intended to satisfy the affirmative defense of Rule 10b5-1(c) under the Exchange Act. Ms. Johnson’s plan provides for, subject to certain market price limits and other terms set forth in the plan, (a) the sale of up to 29,000 shares of Class A common stock, and (b) the exercise of up to 30,000 stock options and sale of the underlying shares of Class A common stock, between June 8, 2026 and August 31, 2026.

On March 6, 2026, Shan Riku Sakakibara, our Chief Product Officer, entered into a pre-arranged stock trading plan. Such trading plan is intended to satisfy the affirmative defense of Rule 10b5-1(c) under the Exchange Act. Ms. Sakakibara’s plan provides for, subject to certain market price limits and other terms set forth in the plan, (a) the sale of up to 20,000 shares of Class A common stock, and (b) the exercise of up to 8,000 stock options and sale of the underlying shares of Class A common stock, between June 8, 2026 and August 31, 2026.

On March 6, 2026, David Tsao, our President, Chief Technology Officer and Director, entered into a pre-arranged stock trading plan. Such trading plan is intended to satisfy the affirmative defense of Rule 10b5-1(c) under the Exchange Act. Mr. Tsao’s plan provides for, subject to certain market price limits and other terms set forth in the plan, the exercise of up to 60,000 stock options and sale of the underlying shares of Class A common stock, between June 15, 2026 and August 28, 2026.

On March 6, 2026, Oguzhan Atay, our Chief Executive Officer and the Chair of our Board of Directors, entered into a pre-arranged stock trading plan. Such trading plan is intended to satisfy the affirmative defense of Rule 10b5-1(c) under the Exchange Act. Mr. Atay’s plan provides for, subject to certain market price limits and other terms set forth in the plan, the exercise of up to 120,000 stock options and sale of the underlying shares of Class A common stock, between June 8, 2026 and August 21, 2026.