A new gene therapy for an ultra-rare disease will have a wholesale cost of $4.25 million, making it the world’s most expensive drug.

The one-time treatment, Lenmeldy, won U.S. regulatory approval on Monday to correct the underlying cause of a hereditary condition called early-onset metachromatic leukodystrophy, or MLD.

MLD is a fatal disease in which infants sometimes start to lose the ability to walk and talk. Orchard Therapeutics said the drug’s price “reflects its clinical, economic and societal value” in a statement Wednesday.

The drug’s cost tops that of CSL Behring LLC’s Hemgenix, a one-time infusion for hemophilia priced at $3.5 million.