The injection of a protein at just the right moment during pregnancy appears to have spared a set of twins—and one other child—from being born without sweat glands.

The daring pregnancy intervention is being described as the first time a drug has been used to treat a developmental disorder in utero.

The experiment, described in a case report today in the New England Journal of Medicine, took place in Germany in 2016 at a clinic that specializes in rare, inherited skin diseases—particular one called XLHED, in which patients are born with fang-like front teeth and without the ability to sweat.

The problem: their bodies don’t produce a specific protein required to make sweat glands.

The German clinic, at the University of Erlangen-Nürnberg, had already participated in a clinical study testing a protein replacement treatment in young children.

But the drug did nothing for the children, the study was abandoned, and the drug maker, Edimer Pharmaceuticals, shut down.

Enter Corinna T., a German nurse who asked that her name be withheld for privacy. She had already had one son with the disease, which she learned of when he was two.

“He was crying incessantly because he had become too hot,” she recalls. Affected children later learn how to take care of themselves, by lying on cool tile floors or dousing themselves with water.

“It can be life-threatening when they are young. They got hot very fast if you leave them in a car,” says Holm Schneider, the physician who counseled Corinna. “But when they are bigger, they instinctively know to cool off.”

Corinna says she then became pregnant with twins, and at 21 weeks into her pregnancy, an ultrasound revealed that they had the same disorder.