The FDA approved Ultragenyx’s gene therapy Fayuvi (rebisufligene etisparvovec) for Sanfilippo syndrome type A, positioning it as the first disease-modifying treatment for the ultra-rare pediatric neurodegenerative disorder. The approval follows a prior complete response letter in 2025 that cited manufacturing-process concerns. Fayuvi is a one-time, AAV-delivered therapy intended to restore SGSH gene function and reduce the buildup of heparan sulfate in the brain. In an open-label, single-arm pediatric study, Ultragenyx reported maintained or improved cognitive outcomes versus historical controls, including a 23.2-point cognitive benefit in earlier-stage/younger patients. Ultragenyx also disclosed a safety profile with liver-enzyme elevations and cytopenias among reported adverse events, along with boxed warnings and label cautions typical of gene therapy products. The FDA’s decision provides a new commercial and clinical reference point for other lysosomal-storage programs awaiting manufacturing and efficacy evidence.