The HdhQ111 knock-in mice exhibit accurate expression of mutant huntingtin from CAG repeats inserted into the mouse HD gene homologue (Htt, formally Hdh) and heterozygous mutant mice display phenotypes at a young age that are dominant, Htt CAG repeat length-dependent and manifest in medium spiny neurons (MSNs) (Wheeler et al. 2000, 2002), conforming to the key genetic features of the HD mutation. This evidence concerns the gene HTT and Huntington disease.