X0306 4/A 2019-03-13 2019-03-14 0 0001501697 X4 Pharmaceuticals, Inc XFOR 0001738081 Mostafa Adam S. C/O X4 PHARMACEUTICALS, INC. 955 MASSACHUSETTS AVENUE, 4TH FLOOR CAMBRIDGE MA 0 1 0 0 See Remarks Stock Option (right to buy) 10.44 2019-03-13 4 A 0 62633 A 2028-10-03 Common Stock 62633 62633 D Twenty-five percent (25%) of the shares subject to the option shall vest on September 5, 2019, and one thirty-sixth (1/36th) of the remaining shares subject to the option shall vest each month thereafter, subject to the Reporting Person continuing to provide service through each such date. This amendment is being filed solely to correct the vesting schedule of the option included in the Form 4 filed on March 14, 2019. Reflects the Reporting Person's stock option to acquire 659,067 shares of X4 common stock for $0.99 per share that was assumed by the Issuer in the merger of Artemis AC Corp., a wholly-owned subsidiary of the Issuer, with and into X4 Therapeutics, Inc. (formerly X4 Pharmaceuticals, Inc.) ("X4") on March 13, 2019 (the "Merger"). Chief Financial Officer and Treasurer All share and per share amounts of the Issuer's common stock and stock options reported in this Form 4 reflect the 1-for-6 reverse stock split of the Issuer's common stock effected on March 13, 2019 after the completion of the Merger. /s/ Adam S. Mostafa 2019-03-15