HTT and Huntington disease: The HEK cell lines expressed mouse Htt exon 1 under control of the endogenous Htt promoter.27 A highly similar construct was used to generate the R6/2 mouse model, which is the murine model of HD with the most pronounced and rapidly progressing phenotype.54 We hypothesized that the extreme pathogenicity of exon 1 HTT represented the maximum possible deviation from normal physiology and thus would exhibit the most pronounced phenotypes.