Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) mitochondrial membrane protein-associated neurodegeneration ?
    ### Response:
    Mitochondrial membrane protein-associated neurodegeneration (MPAN) is a disorder of the nervous system. The condition typically begins in childhood or early adulthood and worsens (progresses) over time.  MPAN commonly begins with difficulty walking. As the condition progresses, affected individuals usually develop other movement problems, including muscle stiffness (spasticity) and involuntary muscle cramping (dystonia).
    