The development of the motor phenotype in R6/2 (express exon 1 of the human HD gene with ∼1,115–150 CAG repeats) and zQ175-KI (knock-in allele replaces the mouse HTT exon 1 with the human HTT exon 1 sequence, with ∼190 CAG repeats) models occurs early, for instance, through the increased excitability of the spiny projection neurons and the reduced number of primary dendrites, which seem to be partly non-progressive. This evidence concerns the gene HTT and Huntington disease.