The R6/1 and R6/2 lines were developed in 1996 and were the first transgenic mouse models of HD, with each solely expressing exon 1 of the human huntingtin (HTT) gene with 115 and 120–150 CAG repeats respectively thus producing 31% and 75% of endogenous HTT protein [45, 46]. This evidence concerns the gene HTT and Huntington disease.