BillionToOne, Inc.

Disclosure of Trading Arrangements

During the fiscal quarter ended June 30, 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 June 2, 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 September 9, 2026 and December 4, 2026.

On June 5, 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 100,000 stock options and sale of the underlying shares of Class A common stock, between September 14, 2026 and November 27, 2026.

On June 5, 2026, David Tsao, our President, Chief Technology Officer and a member 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. Tsao’s plan provides for, subject to certain market price limits and other terms set forth in the plan, the exercise of up to 70,000 stock options and sale of the underlying shares of Class A common stock, between September 9, 2026 and December 4, 2026.

On June 8, 2026, Thomas Lynch, our General Counsel, 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. Lynch’s plan provides for, subject to certain market price limits and other terms set forth in the plan, the exercise of up to 38,226 stock options and sale of the underlying shares of Class A common stock, between September 8, 2026 and December 9, 2026.

On June 8, 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 12,000 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 September 8, 2026 and December 9, 2026.

On June 9, 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 25,582 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 October 1, 2026 and December 31, 2026, to the extent such trades were not previously executed under Ms. Johnson’s prior stock trading plan entered into on March 6, 2026, which terminates on August 31, 2026