The FDA issued a complete response letter rejecting Regenxbio’s RGX‑121 for Hunter syndrome, the company said. The regulator flagged concerns over the trial design, including the reliance on a natural‑history control arm and the use of heparan sulfate as a surrogate biomarker; Regenxbio plans to resubmit with longer‑term data. The PDUFA review was delayed before the CRL, and the decision follows scrutiny of other rare‑disease gene therapies under the Center for Biologics Evaluation and Research. Regenxbio called the outcome devastating for affected families and said it will work to address the FDA’s comments rapidly while continuing its interactions with regulators.