The FDA approved Ultragenyx’s gene therapy Fayuvi (rebisufligene etisparvovec) for Sanfilippo syndrome type A, marking the first FDA-approved treatment specifically for this ultra-rare, fatal childhood neurodegenerative disorder. The approval follows a prior complete response letter in 2025 tied to manufacturing issues, and uses an accelerated pathway based on an open-label, single-arm pediatric study. Ultragenyx’s one-time intravenous infusion delivers an AAV9 vector carrying a functional copy of the SGSH gene, aiming to restore sulfamidase enzyme activity and reduce toxic heparan sulfate buildup in the body and brain. In the study, younger patients and those with earlier-stage disease showed a 23.2-point cognitive benefit versus historical controls (p<0.0001), alongside improvements across receptive and expressive language and motor functions. The label includes thrombotic microangiopathy warnings and notes potential risks related to gene therapy integrating genetic material into the genome. Patients begin corticosteroids before infusion and continue for eight weeks. Adverse events reported in the trial included elevated liver enzymes, nausea, vomiting, fever, decreased appetite, and cytopenias. The FDA action strengthens Ultragenyx’s gene-therapy franchise as the company adds a second approval since August, giving it additional commercialization momentum and market signaling for AAV-based programs in inherited metabolic and lysosomal disorders.