HD is caused by expansion of a CAG repeat region in exon 1 of the huntingtin (HTT) gene located on chromosome 4 beyond a pathogenic threshold of at least 37 CAGs (Figure 1A), with inheritance of 40 or more CAGs in this stretch associated with 100% disease penetrance (Ross and Tabrizi, 2011; Bates et al., 2015; Saudou and Humbert, 2016). This evidence concerns the gene HTT and Huntington disease.