The mutation associated with HD was found to be an expansion of a polyQ tract at the N-terminal coding region of the huntingtin (HTT) protein due to an expansion of a polymorphic CAG trinucleotide repeat sequences in the mutant huntingtin (mHTT) gene, which leads to progressive deterioration of cognitive and motor functions in patients with HD (No authors listed, 1993; Vonsattel and DiFiglia, 1998; Ross and Tabrizi, 2011). The gene discussed is HTT; the disease is Huntington disease.