REGENXBIO Inc. has announced that the U.S. Food and Drug Administration (FDA) has accepted the Biologics License Application $(BLA.AU)$ for RGX-121, a potential one-time gene therapy treatment for Mucopolysaccharidosis II (MPS II), also known as Hunter syndrome. The FDA has granted Priority Review to the application, setting a Prescription Drug User Fee Act (PDUFA) target action date of November 9, 2025. If approved, RGX-121 could become the first gene therapy for MPS II, offering a significant advancement over existing treatments. The therapy has also received several designations, including Orphan Drug Product and Rare Pediatric Disease, which highlight its potential impact. Upon potential approval, Nippon Shinyaku's subsidiary NS Pharma, Inc. will handle U.S. commercialization, while REGENXBIO will manage commercial manufacturing and supply chain.