United States securities and exchange commission logo August 12, 2020 Paula Ragan, Ph.D. Chief Executive Officer X4 Pharmaceuticals, Inc 955 Massachusetts Avenue, 4th Floor Cambridge, Massachusetts 02139 Re: X4 Pharmaceuticals, Inc Registration Statement on Form S-3 Filed August 7, 2020 File No. 333-242372 Dear Dr. Ragan: This is to advise you that we have not reviewed and will not review your registration statement. Please refer to Rules 460 and 461 regarding requests for acceleration. We remind you that the company and its management are responsible for the accuracy and adequacy of their disclosures, notwithstanding any review, comments, action or absence of action by the staff. Please contact Tim Buchmiller at (202) 551-3635 with any questions. Sincerely, Division of Corporation Finance Office of Life Sciences cc: Courtney T. Thorne, Esq.