Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) myoclonic epilepsy with ragged-red fibers ?
    ### Response:
    Myoclonic epilepsy with ragged-red fibers (MERRF) is a disorder that affects many parts of the body, particularly the muscles and nervous system. In most cases, the signs and symptoms of this disorder appear during childhood or adolescence. The features of MERRF vary widely among affected individuals, even among members of the same family.
    