The FDA approved Ultragenyx’s single-dose gene therapy Fayuvi (rebisufligene etisparvovec) for Sanfilippo syndrome type A, delivering the first disease-modifying treatment for the ultra-rare lysosomal storage disorder. The approval follows an earlier denial tied to manufacturing-process concerns and converts Ultragenyx’s lead asset into a commercially sellable product option. Fayuvi uses an AAV vector to deliver a functional copy of SGSH, designed to restore sulfamidase activity and reduce heparan sulfate buildup in the brain. The label is based on an open-label, single-arm pediatric study, with reported cognitive benefits versus historical controls in younger or earlier-stage patients, alongside improvements in language and motor-related outcomes. The agency’s decision also arrives with important safety framing: the treatment includes warnings related to thrombotic microangiopathy and potential risks associated with gene therapies contributing to tumor development through genetic material integration. Ultragenyx will now proceed under the post-approval requirements typical for accelerated approvals, setting a new benchmark for rare-neurology gene therapy. Separate reporting highlighted the therapy’s headline pricing at $4 million per patient, adding to the policy and access debate that often follows high-cost one-time gene therapies.