HTT and Huntington disease: Based on the way in which mutant huntingtin is incorporated into the mouse genome, genetic models of HD can be broadly separated into three categories: (i) transgenic mice that express toxic N-terminal fragments of human mutant Htt (mHtt) in addition to mouse wild-type Htt, (ii) transgenic mice that express full length human mHtt alongside mouse wild-type Htt, and (iii) knock-in mice with expanded CAG repeats in their endogenous Htt gene [7].