The unstable CAG polyQ repeat disorders include Huntington disease (HD), spinal and bulbar muscular atrophy, X-linked (SMAX1, also known as SBMA/Kennedy disease), dentatorubral-pallidoluysian atrophy (DRPLA), Machado-Joseph disease (MJD, also known as SCA3), and the spinocerebellar ataxias, 1 (SCA1), 2 (SCA2), 7 (SCA7) and 17 (SCA17). The gene discussed is AR; the disease is Dentatorubral pallidoluysian atrophy.