U.S. researchers have found the gene that turns muscle and other connective tissues into bone, imprisoning its victim into a second skeleton.

The disease known as the "stone man syndrome" or fibrodysplasia ossificans progressiva (FOP) begins in childhood and cannot be cured. FOP transforms muscles, ligaments and tendons into plates of bone that cover and lock the joints, making movement impossible.

"The discovery of the FOP gene is relevant to every condition that affects the formation of bone and every condition that effects the formation of the skeleton," said Frederick Kaplan, of the University of Pennsylvania, leader of the research.

FOP is among the rarest diseases caused by a genetic mutation and affects about 1 in 2 million individuals, or an estimated 2,500 people. Scientists first found that FOP is caused by a mutated gene that affects bone morphogenetic proteins, controlling the formation and repair of the skeleton.

This discovery led them to a gene called ACVR1, which controls one of the 3 main receptors for morphogenetic proteins, which determine how cells respond to its signals.

By uncovering this gene, researchers will improve understanding and treatment of a wide range of more common skeletal conditions, among which osteoporosis, spinal injuries and sports injuries. The gene could also lead to the development of bone in the laboratory, for treating fractures that fail to heal and skeletal malformations.