Ultragenyx Pharmaceutical received FDA approval for FAYUVI, also known as UX111 and rebisufligene etisparvovec-hopf, a one-time AAV9 gene therapy for children with mucopolysaccharidosis type IIIA, or Sanfilippo syndrome Type A. The company set a U.S. wholesale acquisition cost of $3.95 million per patient and received a Priority Review Voucher, an asset that has traded for about $200 million. FAYUVI delivers a functional SGSH gene to restore production of the enzyme missing in patients, whose inability to break down heparan sulfate contributes to progressive cognitive, speech and motor decline. The FDA had rejected the treatment in 2025 over manufacturing issues before Ultragenyx resubmitted its application. In the pivotal study, treated children aged 2 to 5 maintained or improved cognitive function and recorded a 23.5-point advantage on a cognitive measure versus untreated historical controls. The approval is Ultragenyx's second gene therapy clearance and sixth FDA authorization. FAYUVI is expected to reach qualified U.S. treatment centers within 30 to 60 days, targeting an estimated 300 to 400 identified U.S. patients. William Blair maintained a Market Perform rating and projected $325 million in peak FAYUVI sales. Ultragenyx shares rose about 13% in regular Thursday trading after reaching a 52-week low of $12.73 the previous day.