Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Muscular Dystrophy ?
    ### Response:
    The muscular dystrophies (MD) are a group of more than 30 genetic diseases characterized by progressive weakness and degeneration of the skeletal muscles that control movement. Some forms of MD are seen in infancy or childhood, while others may not appear until middle age or later.
    