The FDA has approved a drug from Denali Therapeutics for Hunter syndrome, a rare genetic disease. The agency's decision is notable because the FDA has recently taken a tougher stance on drugs for rare diseases. Last month, the FDA rejected Regenxbio's Hunter syndrome gene therapy, saying the company needed more clinical data. Advocates worried about the fate of Denali's drug, called Avlayah. In other news, Denali experienced a delay in the review of its drug tividenofusp (tivi), an enzyme replacement therapy for Hunter syndrome. The FDA moved the decision deadline from January 5, 2026 to April 5, 2026 due to updated clinical pharmacology information. The application was filed based on a phase 1/2 study in 47 boys aged around 5 years, where after 24 weeks, levels of the biomarker heparan sulfate in cerebrospinal fluid and urine decreased by an average of 90% from baseline.[1][2]