Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Leukodystrophies ?
    ### Response:
    The leukodystrophies are rare diseases that affect the cells of the brain. Specifically, the diseases affect the myelin sheath, the material that surrounds and protects nerve cells. Damage to this sheath slows down or blocks messages between the brain and the rest of the body.
    