HTT and Huntington disease: Huntington disease (HD) is a progressive dominantly inherited neurodegenerative disease caused by abnormally expanded cytosine-adenine-guanine (CAG) trinucleotide repeats in the first exon in the huntingtin (HTT) gene (The Huntington’s Disease Collaborative Research Group, 1993), which encodes an anomalously expanded polyglutamine (polyQ) tract in the huntingtin protein.