So far, thirteen trinucleotide (TNR) disorders have been found to be caused by expansion of a CAG•CTG tract, including Huntington disease (HD), HD-like 2 (HDL2), myotonic dystrophy type 1 (DM1), spinal and bulbar muscular atrophy (SBMA), dentatorubral-pallidoluysian atrophy (DRPLA), and the spinocerebellar ataxias SCA1, SCA2, SCA3, SCA6, SCA7, SCA8, SCA12, and SCA17 [2], [3], [4]. The gene discussed is CACNA1A; the disease is Huntington disease.