HTT and Huntington disease: In HD, the mutated protein is the ubiquitously expressed protein huntingtin (htt) and the mutation consists of an expansion of a CAG repeat located in the 5′ terminus of the htt gene (HDCRG, 1993) which translates into a polyQ in the N’ terminus of the protein (Gusella et al., 1993; Locke et al., 1993).