For the first time, doctors have used gene therapy to stave off a fatal degenerative brain disease, an achievement that some experts had thought impossible.

The key to making the therapy work? One of medicine’s greatest villains: HIV.

The patients were children who had inherited a mutated gene causing a rare disorder, adrenoleukodystrophy, or ALD. Nerve cells in the brain die, and in a few short years, children lose the ability to walk or talk.

They become unable to eat without a feeding tube, to see, hear or think. They usually die within five years of diagnosis.

The disease strikes about one in 20,000 boys; symptoms first occur at an average age of 7. The only treatment is a bone-marrow transplant — if a compatible donor can be found — or a transplant with cord blood, if it was saved at birth.