BillionToOne, Inc.

Disclosure of Trading Arrangements

During the fiscal quarter ended September 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 August 6, 2026, Shan Riku Sakakibara, our Chief Product Officer, terminated a pre-arranged stock trading plan entered into on June 8, 2026. On August 25, 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 18,000 shares of Class A common stock, and (b) the exercise of up to 10,000 stock options and sale of the underlying shares of Class A common stock, between January 4, 2027 and February 26, 2027.

On September 3, 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 90,000 stock options and sale of the underlying shares of Class A common stock, between December 14, 2026 and February 26, 2027.

On September 8, 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 50,000 stock options and sale of the underlying shares of Class A common stock, between December 14, 2026 and March 11, 2027.

On September 8, 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 6,300 shares of Class A common stock between December 14, 2026 and February 26, 2027.

On September 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 9,420 stock options and sale of the underlying shares of Class A common stock, between December 10, 2026 and March 9, 2027. The description of Mr. Lynch’s prior pre-arranged stock trading plan, as disclosed in the Company’s Quarterly Report on Form 10-Q filed on August 5, 2026, inadvertently underreported the number of shares subject to such plan by 16,886 shares of Class A common stock. Mr. Lynch’s June 8, 2026 plan provided for, subject to certain market price limits and other terms set forth in the plan, the exercise of up to 55,112 stock options and sale of the underlying shares of Class A common stock, between September 8, 2026 and December 9, 2026. All shares subject to the plan had been sold as of September 17, 2026, as reported in Mr. Lynch’s Form 4 filings.

On September 8, 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, the exercise of up to 30,000 stock options and sale of the underlying shares of Class A common stock, between January 4, 2027 and March 31, 2027.