Several disorders such as Huntington disease, myotonic dystrophy and spinocerebellar ataxia are caused by an excessive number of trinucleotide repeats.19–23 For example, the expansion of triplet repeats in the human IT15 gene leads to Chorea Huntington Disease (HD).24,25 Alleles with less than 26 repeats are considered normal and are stable, but alleles that contain 27–36 triplet repeats may expand during meiosis, hence leading to increased repeats in the next generation. The gene discussed is HTT; the disease is Huntington disease.