United States securities and exchange commission logo October 21, 2020 Paula Ragan, Ph.D. Chief Executive Officer X4 Pharmaceuticals, Inc 61 North Beacon Street, 4th Floor Boston, MA 02134 Re: X4 Pharmaceuticals, Inc Registration Statement on Form S-3 Filed October 19, 2020 File No. 333-249544 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 Ada D. Sarmento at 202-551-3798 with any questions. Sincerely, Division of Corporation Finance Office of Life Sciences cc: Daniel I. Goldberg, Esq.