HTT and Huntington disease: In the most common polyQ disorders, HD and SCA3, the expression of at least 36 CAG repeats in the first exon of the huntingtin (HTT) gene and of at least 60 CAG repeats in the 10th exon of the ataxin 3 (ATXN3) gene, respectively, is sufficient to cause pathogenic effects (Orr and Zoghbi, 2007).