Fibrodysplasia ossificans progressiva (FOP) is a disorder in which muscle tissue and connective tissue such as tendons and ligaments are gradually replaced by bone (ossified), forming bone outside the skeleton (extra-skeletal or heterotopic bone) that constrains movement. This process generally becomes noticeable in early childhood, starting with the neck and shoulders and proceeding down the body and into the limbs.
Fibrodysplasia ossificans progressiva is an ultra rare disorder and is believed to occur in approximately 1 in 2 million people worldwide, with only several hundred cases having been reported.