Scientists have discovered a mutant gene that triggers the body to form a second, renegade skeleton, solving the mystery of a rare disease called FOP that imprisons children in bone for life.

The finding, reported Sunday, may one day lead to development of a drug, not only to treat the rare bone disorder, but more common bone buildup related to head and spine trauma, and even sports injuries, the researchers said.

"We've reached the summit," said Dr. Frederick Kaplan, an orthopedist whose team at the University of Pennsylvania School of Medicine pinpointed the cause of FOP, or fibrodysplasia ossificans progressiva. The disease is believed to afflict only 2,500 people worldwide.

The research was reported in the online edition of the journal Nature Genetics by Kaplan, geneticist Eileen M. Shore, and their Penn colleagues, with contributions from researchers in Australia, Brazil, France, Germany, Great Britain, the Netherlands and South Korea.

After 15 years of work involving study of the genetic makeup of multigenerational families around the world, scientists at Penn's Center for Research in FOP and Related Disorders found that FOP is caused by a single mutation in a gene called ACVR1. This devastating glitch means that tendons, ligaments and skeletal muscle begin painfully transforming into bone, sometimes locking joints overnight.

The genetic twist that leads to FOP, Kaplan said in a telephone interview, "is relevant to every condition that affects the formation of bone and every condition that affects the formation of the skeleton."

The researchers believe it should be possible to develop a drug that would block or bypass the genetic trigger of the extra bone growth. Eventually, it might block the unneeded bone that occasionally forms after hip-replacement surgery.

"In the next five years, this might open up the possibility of developing drugs that would be effective in stopping bone formation," said Dr. Victor A. McKusick, a genetics pioneer and professor of medical genetics at Johns Hopkins University School of Medicine in Baltimore.

He said the FOP genetic breakthrough is likely to shed light on other related diseases.

"The first thing that comes to mind is osteoporosis, which is the flip side of the coin when it comes to bone formation," he said. "When one learns about one side — extra bone growth — it helps you understand what goes the other way" — bone breakdown.

Stephanie Snow, a 15-year-old with FOP, hopes the finding will lead to a drug that can stop the stiffening damage to her body, which includes a fused, immobile neck, arms she can't raise and problems with hip mobility. The Santa Maria, Calif. teenager dreams of becoming a veterinarian.

"If they develop a pill we can take every day, I can move and do more things, and it might be easier for me to become a vet, like I've always wanted to," she said.

FOP patient Jeannie Peeper, 47, of Winter Springs, Fla., is totally immobilized but still a leader in the International FOP Association that represents patients and their families.

"The gene discovery is an extraordinary gift to the FOP community and a monumental milestone on our road to a cure," she said.