HTT and Huntington disease: Huntington’s disease (HD) is a devastating, autosomal-dominant inherited, neurodegenerative disorder characterized by progressive motor deficits, cognitive impairments, and neuropsychiatric symptoms.[1] It is caused by a cytosine-adenine-guanine (CAG) repeat expansion in the first exon of the Huntingtin gene, located in the short arm of chromosome 4 (4p16.3),[2] encoding for a mutant huntingtin (mHTT) protein.