Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Parkinson disease ?
    ### Response:
    Parkinson disease belongs to a group of conditions called movement disorders. The four main symptoms are tremor, or trembling in hands, arms, legs, jaw, or head; rigidity, or stiffness of the limbs and trunk; bradykinesia, or slowness of movement; and postural instability, or impaired balance.
    