ATXN2 and Huntington disease: Although several genetic factors have been proposed as modifiers, such as CAG repeat size of normal ATXN3 (SCA3), HTT (HD), ATXN2 (SCA2) and ATN1 (DRPLA) alleles, APOE status, and expression level of HSP40 [4–6], these were not replicated by subsequent studies [7, 8].