Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the genetic changes related to Huntington disease ?
    ### Response:
    Mutations in the HTT gene cause Huntington disease. The HTT gene provides instructions for making a protein called huntingtin. Although the function of this protein is unknown, it appears to play an important role in nerve cells (neurons) in the brain.  The HTT mutation that causes Huntington disease involves a DNA segment known as a CAG trinucleotide repeat.
    