Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Spinal Muscular Atrophy ?
    ### Response:
    Spinal muscular atrophy (SMA) is a genetic disease that attacks nerve cells, called motor neurons, in the spinal cord. These cells communicate with your voluntary muscles - the ones you can control, like in your arms and legs. As the neurons die, the muscles weaken.
    