In HD, up to 75% of the variability in AAO can be explained by the HTT CAG repeat length (Hmida-Ben Brahim et al., 2014), while in SCA1, SCA2, SCA3, SCA6, and SCA7, the CAG repeat in the causative gene explains between 32 and 80% of the AAO variability (Tezenas et al., 2014). The gene discussed is CACNA1A; the disease is Huntington disease.