HTT and Huntington disease: In healthy individuals, the lengths for the huntingtin gene (CAG)n repeats range from n ∼ 10–30, yet when they expand to n >37 repeats, overt and progressive HD symptoms become apparent (La Spada et al., 1991; MacDonald et al., 1993; Mirkin, 2007; Nalavade et al., 2013; Usdin et al., 2015; Richards, 2016; Polyzos and McMurray, 2017).