Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Pelizaeus-Merzbacher Disease ?
    ### Response:
    Pelizaeus-Merzbacher disease (PMD) is a rare, progressive, degenerative central nervous system disorder in which coordination, motor abilities, and intellectual function deteriorate. The disease is one of a group of gene-linked disorders known as the leukodystrophies, which affect growth of the myelin sheath -- the fatty covering that wraps around and protects nerve fibers in the brain.
    