X0306 4 2020-09-10 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 2020-09-10 4 M 0 13860 2.00 A 13860 D Common Stock 2020-09-10 4 S 0 13860 41.5844 D 0 D Common Stock 2020-09-11 4 M 0 13571 2.00 A 13571 D Common Stock 2020-09-11 4 S 0 13571 40.3309 D 0 D Common Stock 2020-09-14 4 M 0 22569 2.00 A 22569 D Common Stock 2020-09-14 4 S 0 22569 41.0043 D 0 D Common Stock 646000 I By GAD Enterprises LLC Common Stock 1030356 I By WG Biotech ApS Employee Stock Option (right to buy) 2.00 2020-09-10 4 M 0 13860 0 D 2025-06-10 Common Stock 13860 486140 D Employee Stock Option (right to buy) 2.00 2020-09-11 4 M 0 13571 0 D 2025-06-10 Common Stock 13571 472569 D Employee Stock Option (right to buy) 2.00 2020-09-14 4 M 0 22569 0 D 2025-06-10 Common Stock 22569 450000 D This transaction was executed in multiple trades at prices ranging from $42.77 to $41.00. The price reported reflects the weighted average sale price for this cashless exercise of 13,860 stock options expiring June 10, 2025. The Reporting Person hereby undertakes to provide upon request to the SEC staff, the issuer or a security holder of the Issuer full information regarding the number of shares and prices at which the transaction was effected. This transaction was executed in multiple trades at prices ranging from $40.79 to $40.00. The price reported reflects the weighted average sale price or this cashless exercise of 13,571 stock options expiring June 10, 2025. The Reporting Person hereby undertakes to provide upon request to the SEC staff, the issuer or a security holder of the Issuer full information regarding the number of shares and prices at which the transaction was effected. This transaction was executed in multiple trades at prices ranging from $40.42 to $41.47. The price reported reflects the weighted average sale price or this cashless exercise of 22,569 stock options expiring June 10, 2025. The Reporting Person hereby undertakes to provide upon request to the SEC staff, the issuer or a security holder of the Issuer full information regarding the number of shares and prices at which the transaction was effected. 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. WG Biotech ApS owns 5,508,392 shares of the Issuer's common stock. GAD Enterprises, LLC (of which Reporting Person is sole member) owns approximately 18.71% of WG Biotech ApS but has no voting or dispositive power over the shares of common stock held by WG Biotech ApS. The stock option vested and became exercisable as to 25% of the shares subject to the option on June 10, 2016, and thereafter vested as to 1/48th of the shares in equal monthly installments, until such time as the option was 100% vested on June 10, 2019. /s/ Thomas Gad 2020-09-15