Huntington's disease is caused by a mutation of the hd gene, which contains extra copies of the nucleotide sequence which encodes extras copies of the amino acid into the protein huntingtin.

Huntington's disease is caused by a mutation of the HD gene, which contains extra copies of the nucleotide sequence CAG, which encodes extra copies of the amino acid glutamine into the protein Huntingtin. Mutations in the HTT gene causes Huntington disease. The HTT gene provides instructions for making a protein called huntingtin. This protein appears to play an important role in nerve cells in the brain. The HTT mutation that causes the disease involves a DNA segment known as CAG trinucleotide repeat. In one  part of the chain of amino acids in Huntingtin protein there is a long string of amino acid glutamine.