X0306 4 2020-04-13 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 2020-04-13 4 S 0 4000 29.4278 D 832000 I By GAD Enterprises LLC Common Stock 1030356 I By WG Biotech ApS Represents shares sold pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on April 11, 2019. This transaction was executed in multiple trades at prices ranging from $30.38 to $28.62. The price reported reflects the weighted average sale price. 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. /s/ Thomas Gad 2020-04-15