CACNA1A and Huntington disease: CAG-type expansion disorders include spinal and bulbar muscular atrophy (SBMA) [4], dentatorubral-pallidoluysian atrophy (DRPLA) [6], Huntington disease (HD) [7] and a range of spinocerebellar ataxias (SCAs) including SCA1 [8], SCA2 [9], SCA3 [10], SCA6 [11], and SCA7 [12].