In addition to variation in the number of CAG repeats in causative genes, the variability of AO in PolyQ diseases can also be explained by the effects of modifier sister genes: ATXN2, ATN1 and HTT in SCA3/MJD; ATXN1 and ATXN3 in SCA6; and ATXN3 and TBP in SCA7 [7], as well as other modifiers like APOE in SCA3/MJD [11, 12]. The gene discussed is HTT; the disease is Spinocerebellar ataxia type 3.