U.S. regulators have approved the most expensive medicine ever, for a disorder that destroys a baby's muscle control and kills nearly all of those with the most common variant of the disease within a couple years.

The treatment is priced at $2.125 million US. Out-of-pocket costs for patients will vary based on insurance coverage.

The gene therapy, developed by Novartis, treats a rare condition called spinal muscular atrophy, which strikes about 400 babies born in the U.S. each year.

The U.S. Food and Drug Administration on Friday approved the drug, called Zolgensma, for all children under age 2 who are confirmed by a genetic test to have any of the four types of the disease.

The therapy, a one-time infusion that takes about an hour, will be available within two weeks.

Novartis said it will let insurers make payments over five years, at $425,000 per year, and will give partial rebates if the drug doesn't work.

The one other treatment for the disease approved in the U.S., called Spinraza, is not a one-time injection. Instead, it must be given every four months after initial treatment. Biogen, Spinraza's maker, charges a list price of $750,000 for the first year and then $350,000 per year after that.

In patient testing, babies with the most severe form of the disease who got the treatment within 6 months of birth had limited muscle problems. Those who got the treatment earliest did best.

Rebecca Van Fraassen’s family is just one many advocating for government help in paying for treatments for rare diseases, some of which can reach above $1 million per year. 2:47

Babies given Zolgensma after six months stopped losing muscle control, but the medicine was unable to reverse damage already done.

Novartis has been working with states to get genetic testing for newborns required at birth and expects most states will have that requirement by next year.