HTT and juvenile Huntington disease: In Huntington’s disease (HD), an expansion of a CAG repeat within exon 1 of the huntingtin (HTT) gene, which produces a huntingtin (HTT) protein with an expanded polyglutamine (polyQ) repeat, leads to a progressive and fatal neurodegenerative pathology (Ross & Tabrizi, 2011; Bates et al, 2015; McColgan & Tabrizi, 2018).