Fibrodysplasia ossificans progressiva is caused by a mutation of the gene ACVR1. The mutation affects the body's repair mechanism, causing fibrous tissue including muscle, tendons, and ligaments to be ossified, either spontaneously or when damaged as the result of trauma. In many cases, otherwise minor injuries can cause joints to become permanently fused as new bone forms and replaces the damaged muscle tissue. This new bone formation (known as "heterotopic ossification") eventually forms a secondary skeleton and progressively restricts the patient's ability to move. Bone formed as a result of this process is identical to "normal" bone, simply in improper locations. Circumstantial evidence suggests that the disease can cause joint degradation separate from its characteristic bone growth.