X0306 4 2020-11-03 0 0001722964 Y-mAbs Therapeutics, Inc. YMAB 0001106519 GILL DAVID N C/O Y-MABS THERAPEUTICS, INC. 230 PARK AVENUE SUITE 3350 NEW YORK NY 10169 1 0 0 0 Common Stock 2020-11-03 4 M 0 4000 11.16 A 4000 D Common Stock 2020-11-03 4 S 0 4000 45.00 D 0 D Non-Employee Stock Option (right to buy) 11.16 2020-11-03 4 M 0 4000 0 D 2028-04-24 Common stock 4000 28000 D The option exercise and sale transaction reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan previously adopted by the Reporting Person. Stock option granted to the Reporting Person as a non-employee director pursuant to the 2015 Equity Incentive Plan of Y-mAbs Therapeutics, Inc. The stock option was (a) granted on April 24, 2018 pursuant to the 2015 Equity Incentive Plan of Y-mAbs Therapeutics, Inc., and (b) represents the right to purchase a total of 32,000 shares of common stock; and (c) vests and becomes exercisable in equal monthly installments until the third anniversary of the date of grant subject to continued service through such dates and unless vesting is accelerated pursuant to the terms of the grant. /s/ David N Gill 2020-11-05