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Important Notice Regarding the Availability of Proxy Materials for the Stockholder Meeting ofY-MABS THERAPEUTICS, INC.To Be Held On:June 8, 2023 at 10:00 a.m. Eastern TimeFor Stockholders of Record as of April 13, 2023via the Internet at https://web.lumiagm.com/257688918 (password: ymabs2023) COMPANY NUMBERACCOUNT NUMBERCONTROL NUMBERThis communication presents only an overview of the more complete proxy materials that are available to you on the Internet. Weencourage you to access and review all of the important information contained in the proxy materials before voting.If you want to receive a paper or e-mail copy of the proxy materials you must request one. There is no charge to you for requesting acopy. To facilitate timely delivery please make the request as instructed below before 5/25/2023.Please visit http://www.astproxyportal.com/ast/22412, where the following materials are available for view:• Notice of Annual Meeting of Stockholders• Proxy Statement• Form of Electronic Proxy Card• Annual Report on Form 10-KTO REQUEST MATERIAL: TELEPHONE: 888-Proxy-NA (888-776-9962) 718-921-8562 (for international allers)E-MAIL: [email protected]: https://us.astfinancial.com/OnlineProxyVoting/ProxyVoting/RequestMaterialsTO VOTE: ONLINE: To access your online proxy card, please visit www.voteproxy.com and follow the on-screeninstructions or scan the QR code with your smartphone. You may enter your voting instructions atwww.voteproxy.com up until 11:59 PM Eastern Time the day before the cut-off or meeting date.VIRTUALLY AT THE MEETING: The company will be hosting the meeting live via the Internet this year.To attend the meeting via the Internet please visit https://web.lumiagm.com/257688918(password: ymabs2023) and be sure to have available the control number.TELEPHONE: To vote by telephone, please visit www.voteproxy.com to view the materials and to obtainthe toll free number to call.MAIL: You may request a card by following the instructions above. 1. Election of irectors:James I. Healy, M.D.Ashutosh Tyagi, M.D.Laura J. Hamill2. To ratify the appointment of PricewaterhouseCoopers LLP as our independentregistered public accounting firm for our fiscal year ending December 31, 2023.3. To approve, on a non-binding advisory vote basis, the compensation of the Company’snamed executive officers.4. The transaction of such other business as may properly come before the meeting.THE BOARD RECOMMENDS A VOTE “FOR” THE ELECTION OF ALL DIRECTORS, AND“FOR” ITEMS 2 AND 3. Please note that you cannot use this notice to vote by mail.