Huntington’s disease (HD) is a heritable, fully penetrant, autosomal neurodegenerative disease caused by a CAG trinucleotide expansion within the first exon of the Huntingtin (HTT) gene (Macdonald, 1993), leading to an expanded polyglutamine (polyQ) stretch at the N-terminal portion of the encoded HTT protein. The gene discussed is HTT; the disease is Huntington disease.