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 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.
    