Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Primary Lateral Sclerosis ?
    ### Response:
    Primary lateral sclerosis (PLS) is a rare neuromuscular disease with slowly progressive weakness in voluntary muscle movement. PLS belongs to a group of disorders known as motor neuron diseases. PLS affects the upper motor neurons (also called corticospinal neurons) in the arms, legs, and face.
    