Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Fibrodysplasia ossificans progressiva ?
    ### Response:
    Fibrodysplasia ossificans progressiva (FOP) is a disorder in which skeletal muscle and connective tissue, such as tendons and ligaments, are gradually replaced by bone (ossified). This condition leads to bone formation outside the skeleton (extra-skeletal or heterotopic bone) that restricts movement.
    