HD is caused by a CAG trinucleotide repeat expansion mutation (>35 units) that produces an elongated version of a normally polymorphic polyglutamine segment in huntingtin [3]–[5], a large 350 kDa ubiquitously expressed HEAT (huntingtin, elongation factor 3, the A subunit of protein phosphatase 2A, and TOR1) repeat protein. Here, HTT is linked to Huntington disease.