Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Huntington disease ?
    ### Response:
    What causes Huntington disease? Huntington disease (HD) is caused by a change (mutation) in the HTT gene. This gene gives instructions for making a protein called huntingtin. The exact function of this protein is unclear, but it appears to be important to nerve cells (neurons) in the brain.
    