HTT and Huntington disease: To evaluate the impact of expressing a version of wild-type htt lacking its short polyQ stretch on the motor and behavioral phenotypes exhibited by a mouse model for HD, HdhΔQ/+ mice were crossed with the CAG140 knock-in mouse expressing full-length htt with a chimeric human/mouse htt exon 1 containing an expanded stretch of 140 glutamines [25], (for a diagram of the knockin alleles used in this study, see Figure 1A).