ATXN1 and Huntington disease: In contrast to HD where repeat expansion in striatal MSNs drives onset of disease, data presented here along with previous data indicate that, in SCA1, expanded ATXN1 expression in the cerebellum drives disease onset and that striatal ATXN1 repeat expansion only contributes to age-dependent progression of motor deficits well after disease onset (Figure 3B and Supplemental Figure 4C).