HTT and juvenile Huntington disease: PolyQ diseases include Huntington’s disease (HD), six spinocerebellar ataxias (SCAs) dentatorubral-pallidoluysian atrophy (DRPLA), and spinobulbar muscular atrophy (SBMA), each with its own causative gene: HTT, ATXN1, ATXN2, ATXN3, CACNA1A, ATXN7, TBP, ATN1, and AR, respectively (Williams and Paulson, 2008).