ATXN3 and Huntington disease: Despite the fact that SCA3 is much rarer than HD, the higher average rate of somatic expansion in SCA3, coupled with the lack of a major effect for repeat length, and the clear effect of age on somatic expansion of the ATXN3 repeat in individuals with SCA3, suggest that SCA3 might present as an attractive disorder in which to establish proof-of-action in a clinical trial of a somatic expansion supressing drug with peripheral exposure.