The R6/2 mouse is transgenic for a genomic DNA fragment encoding exon 1 of human HTT and is a model of the incomplete splicing event that occurs in all HD knock-in mouse models, YAC128 and BACHD mice17, and in HD patient tissues18, resulting in an exon 1 – intron 1 mRNA that in all cases produces the highly pathogenic exon 1 HTT protein. Here, HTT is linked to Huntington disease.