HTT and Huntington disease: Genetic models of HD have also been studied, including the R6/2 mouse, which is transgenic for a 150 repeat trinucleotide CAG expansion in exon 1 of HTT and manifests severe neurodegeneration, neuronal intranuclear inclusions, and sensorimotor deficits that result in a severely shortened survival of less than 4 months of age (Mangiarini et al., 1996; Davies et al., 1997).