HTT and Huntington disease: HD has autosomal dominant heredity and is caused by an expanded Cytosine-Adenine-Guanine (CAG) repeat (≥36) in the huntingtin coding gene (IT15) on chromosome 4, resulting in the expression of mutant huntingtin protein (mHTT) with abnormal number of glutamine repeats (polyQ) in its N terminus [32,34].