WEDNESDAY, May 8 (HealthDay News) -- Researchers say they finally know what causes babies to be born with port-wine stain birthmarks and a rarer but related condition that often leads to lifelong struggles with blindness, seizures and mental disabilities.

In a new study published in the May 8 issue of The New England Journal of Medicine, scientists say a single random change to a single gene after conception causes both the birthmarks, which affect about one in 300 babies, and Sturge-Weber Syndrome, which occurs in about one in 20,000 births.

The change causes a molecular switch that's normally flipped on and off by chemical messages received by the cell to get stuck in the 'on' position.

"It's great because we have an immediate biochemical understanding of what's happening, and that means we can immediately move on to the idea of what to do about it," said Jonathan Pevsner, director of bioinformatics at the Kennedy Krieger Institute in Baltimore.

"Previously, it was sort of like walking in the dark. We had to settle for treating symptoms and making best guesses," said study co-author Dr. Anne Comi, director of the Kennedy Krieger Institute's Hunter Nelson Sturge-Weber Center. "This turns on a light to guide steps for research and treatment, and the direction for where we need to go."

The discovery marks a milestone for the field of genetics, where advances have only recently made it possible to find these rare "lightning strike" mutations.

It is also a triumph for parents of children with the rare disorder who began saving samples of skin and brain tissue years ago, before the technology was even in place to test them.

"I started laughing. I was just in shock," said Karen Ball, president and CEO of the Sturge-Weber Foundation, which has more than 5,000 members around the world. "I was just like, 'Oh, my gosh, it worked.' It's a giddy rush of emotions."

Ball and her husband started the Sturge-Weber Foundation in 1987, after their daughter, Kaelin, was born with the condition.