Fibrodysplasia ossificans progressiva (FOP) is an extremely rare genetic disorder where muscle tissue and connective tissue are gradually replaced by bone. It is caused by a mutation in the body's repair mechanism, causing extra bone to form following injuries. There is no treatment for FOP, and surgery can make it worse by triggering more bone growth. Most cases are caused by spontaneous genetic mutations, and those affected have very limited mobility as the condition progresses and more joints become frozen in place by extra bone formation.