X0306 4 2020-05-13 0 0001722964 Y-mAbs Therapeutics, Inc. YMAB 0001752940 Kruse Bo C/O Y-MABS THERAPEUTICS, INC. 230 PARK AVENUE, SUITE 3350 NEW YORK NY 10169 0 1 0 0 EVP, CFO, Secy & Treas. Common Stock 2020-05-13 4 S 0 14724 37.8645 D 285276 D Common Stock 2020-05-13 4 S 0 19230 37.8783 D 37847 I By Investeringsselskabet G.H. ApS Common Stock 2020-05-14 4 S 0 28199 36.0251 D 257077 D Common Stock 2020-05-14 4 S 0 37847 36.0275 D 0 I By Investeringsselskabet G.H. ApS This transaction was executed in multiple trades at prices ranging from $39.13 to $37.50. 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. This transaction was executed in multiple trades at prices ranging from $39.13 to $37.50. 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. Investeringsselskabet G.H. ApS is Reporting Person's personal holding company of which Reporting Person is the sole owner and as such has the sole voting and dispositive power with respect to such shares. This transaction was executed in multiple trades at prices ranging from $36.10 to $36.02. 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. This transaction was executed in multiple trades at prices ranging from $36.34 to $36.02. 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. /s/ Bo Kruse 2020-05-15