The disorder -- X-linked hypohidrotic ectodermal dysplasia -- primarily affects boys and can result in overheating, which can be life-threatening in severe cases. It can also affect saliva and tear production.
Corinna and her husband, Tobias, who requested that CNN not use their last name for privacy reasons, discovered the genetic defect when their son was 2 years old and still had no teeth.
"The hotter it is, the more water he needs to cool down his body temperature," Corinna, 40, who lives in Germany, wrote in an email. It's like "the engine of a car that runs without cooling water."
Then, Corinna and Tobias found out that she was pregnant again -- with twin boys. Doctors confirmed that the twins carried the affected gene.
"To be honest, that was what I had almost expected," she said.
By then, Corinna had been in touch with Dr. Holm Schneider, a professor of pediatrics at University Hospital Erlangen who had treated mice and dogs with a version of the disorder by injecting a protein into the amniotic fluid. But this had never been done in humans.
"We were fully aware of the fact that our twins would be the first babies in the world to receive this treatment in utero," Corinna said.
Her twins' cases were published, along with that of a third baby boy, in April in the New England Journal of Medicine.
A medical first
"We realized early that this therapy had worked," Corinna said of the sweat tests that doctors conducted after her twins, Linus and Maarten, were born.
"But only when we discovered wet baby car seats in the first summer, we became really excited about the effect of the treatment," she said.
"They can run as they like to do and can enjoy the sun ... as everybody else without overheating," Schneider, head of Germany's Center for Ectodermal Dysplasias, wrote in an email.
The disorder results from the lack of a functional protein called ectodysplasin A, which signals the development of certain tissues in utero. The treatment the doctors used was a "fusion protein," or "protein-replacement therapy," that contained a key part of the functional protein, the authors wrote. Previous efforts to treat babies with the same protein shortly after birth had failed, Schneider said.
"The drug must be administered at the right time point during development," he said.
Within the first few weeks of pregnancy, an embryo splits into three layers. The outermost l