HTT and Huntington disease: HD is a rare, adult-onset, autosomally-dominant neurodegenerative disorder caused by the dynamic expansion of a polymorphic CAG repeat in exon 1 of the huntingtin (HTT) gene that encodes the mutant huntingtin (HTT) protein (2), with an estimated prevalence of 5–17 individuals per 100,000 (3, 4).