X0306 4 2019-12-30 0 0001722964 Y-mAbs Therapeutics, Inc. YMAB 0001753224 Gad Thomas C/O Y-MABS THERAPEUTICS, INC. 230 PARK AVENUE, SUITE 3350 NEW YORK NY 10169 1 1 0 0 President and Chairman Common Stock 860000 I By GAD Enterprises LLC Common Stock 2019-12-30 4 J 0 201 D 1030356 I By WG Biotech ApS Reporting Person is sole member of GAD Enterprises, LLC and the sole member and manager of GAD Management, LLC, the manager of GAD Enterprises, LLC, and as such Reporting Person has sole voting and dispositive power with respect to such shares. Pursuant to a transfer by Weco Group A/S of 498,392 shares of the Common Stock of Y-mAbs Therapeutics, Inc. (the "Company") to WG Biotech ApS in exchange for 9,948 ordinary shares of WG Biotech ApS effective as of December 30, 2019, the number of shares of the Company's Common Stock owned by WG Biotech ApS increased from 5,010,000 to 5,508,392 shares. As a result of the transaction, the percentage interest of GAD Enterprises, LLC (of which Reporting Person is sole member) in WG Biotech ApS decreased from 20.57% to 18.71% and the number of shares of the Company's Common Stock indirectly beneficially owned by GAD Enterprises, LLC decreased from 1,030,557 to 1,030,356. Gad Enterprises, LLC has no voting or dispositive power over the shares of the Company's Common Stock held by WG Biotech ApS. The closing sale price of the Company's Common Stock on December 30, 2019, the date of the transaction, was $31.07. /s/ Thomas Gad 2019-12-31