The FDA issued a complete response letter rejecting Regenxbio’s RGX‑121 gene therapy for Hunter syndrome, citing concerns about the development program’s ability to define the patient population and the reliance on a biomarker endpoint. Regenxbio signaled it will pursue a resubmission with longer‑term data but described the decision as devastating for families of boys with severe disease. Regulatory officials raised questions about the use of heparan sulfate as a surrogate endpoint and the trial’s control strategy, prompting the agency to ask for more robust efficacy data. The rejection follows earlier agency scrutiny and clinical holds on related Regenxbio programs and underscores heightened FDA scrutiny of biomarker‑driven accelerated approvals for one‑time genetic medicines.