HTT and Huntington disease: After identifying through linkage analyses a polymorphic deoxyribonucleic acid (DNA) marker on the fourth chromosome in HD patients [3], the Huntington Disease Collaborative Research Group linked the disease to the IT15 gene (“interesting transcript 15”, subsequently renamed as the huntingtin gene), whose first exon contained a repetitive unstable CAG (cytosine-adenine-guanine) sequence, which was significantly expanded in HD patients [4].