A breakthrough in HD research came in 1983 with the discovery of a genetic marker of the disease near the tip of the short arm of chromosome 4 (Gusella et al., 1983), and 10 years later a triplet expansion repeat in the novel gene Huntingtin (HTT) was mapped as the causal mutation for HD (MacDonald et al., 1993; Holmans et al., 2017). This evidence concerns the gene HTT and Huntington disease.