To understand the mechanisms of HD pathogenesis and to establish experimental platforms for drug discovery, multiple lines of genetically altered mice have been generated that can be classified into three groups: (a) mice expressing truncated human HTT fragments, e.g., R6 lines; (b) mice expressing full-length human HTT modified by the insertion of variable numbers of CAG repeats, e.g., YAC128 or BACHD lines; and (c) knock-in models, in which CAG repeats are inserted into the endogenous mouse Htt gene, e.g., HdhQ92 line (Menalled and Chesselet, 2002; Chang et al., 2015). This evidence concerns the gene HTT and Huntington disease.