When ATTO insiders can sell

The lock-up agreements pertaining to this offering will expire 180 days from the date of this prospectus. However, Morgan Stanley & Co. LLC, Leerink Partners LLC, Citigroup Global Markets Inc. and RBC Capital Markets, LLC may, in their sole discretion, permit our officers, directors and other stockholders to sell shares prior to the expiration of the lock-up agreements. After the lock-up agreements expire, based on 26,029,017 shares outstanding as of March 31, 2026, approximately up to an additional 26,749,461 shares of common stock will be eligible for sale in the public market, approximately 15,044,561 of which shares are held by our officers, directors and their affiliated entities, and will be subject to volume limitations under Rule 144 under the Securities Act.

After the lock-up agreements expire, based on 26,029,017 shares outstanding as of March 31, 2026, approximately up to an additional 26,749,461 shares of common stock will be eligible for sale in the public market, approximately 15,044,561 of which shares are held by our officers, directors and their affiliated entities, and will be subject to volume limitations under Rule 144 under the Securities Act. After this offering, the holders of an aggregate of 24,822,612 shares of our outstanding common stock as of March 31, 2026 will have rights, subject to some conditions, to require us to file registration statements covering their shares or to include their shares in registration statements that we may file for ourselves or our stockholders.

Rule 10b5-1 Plans Our directors, officers and key employees may adopt written plans, known as Rule 10b5-1 plans, in which they will contract with a broker to buy or sell shares of our common stock on a periodic basis. Under a Rule 10b5-1 plan, a broker executes trades under parameters established by the director or officer when entering into the plan, without further direction from the director or officer. The director or officer may amend or terminate a Rule 10b5-1 plan, subject to certain requirements.

…t sales of common stock or other securities acquired in this offering or in such open market transactions; 216 Table of Contents transfers of shares of common stock or any security convertible into common stock as (i) a bona fide gift; or to a charitable organization or educational institution, (ii) to an immediate family member of the holder or to any trust for the direct or indirect benefit of the holder or an immediate family member of the holder, or if the holder is a trust, to a grantor, trustee or beneficiary of the trust (including such beneficiary's estate) of the holder, (iii) to any corporation, partnership, limited liabil…

…other health care professionals (such as physician assistants and certain advanced practice nurses), and teaching hospitals, as well as ownership and investment interests held by the physicians described above and their immediate family members, with the information made publicly available on a searchable website; the Foreign Corrupt Practices Act (FCPA), which prohibits U.S. businesses and their representatives from directly or indirectly offering to pay, paying, promising to pay or authorizing the payment of money or anything of value to a foreign official in order to influence any act or decision of the foreign official in his or…

…le preferred stock as of March 31, 2026 into an aggregate of 21,517,976 shares of our common stock in connection with the completion of this offering, and excludes: 2,704,695 shares of our common stock issuable upon the exercise of stock options outstanding as of March 31, 2026 under our 2023 Equity Incentive Plan (2023 Plan), with a weighted-average exercise price of $4.03 per share; 87,914 shares of our common stock issuable upon the exercise of stock options granted after March 31, 2026 under our 2023 Plan, with a weighted-average exercise price of $9.07 per share; 5,283,264 shares of our common stock reserved for future issuance…