Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) progressive osseous heteroplasia ?
    ### Response:
    Progressive osseous heteroplasia is a disorder in which bone forms within skin and muscle tissue. Bone that forms outside the skeleton is called heterotopic or ectopic bone. In progressive osseous heteroplasia, ectopic bone formation begins in the deep layers of the skin (dermis and subcutaneous fat) and gradually moves into other tissues such as skeletal muscle and tendons.
    