The Food and Drug Administration on Friday approved a gene therapy for spinal muscular atrophy, a rare muscle-wasting disease that, in its most severe form, kills many babies before they turn 2.

The price will be $2.1 million, believed to be highest ever set for a one-time treatment.

The therapy, to be sold as Zolgensma, alters the underlying genetic cause of spinal muscular atrophy and may permanently stop the disease. It is among the first of a host of gene therapies that promise a cure for deadly inherited conditions.

“We feel we’re on a path where we hope one day to be able to bring S.M.A. almost to elimination,” said Vas Narasimhan, the Novartis chief executive, in a conference call with reporters.

Many drugs for rare diseases have arrived on the market recently costing hundreds of thousands of dollars a year. But few have crossed the $1 million threshold.