The Humanized AHR Mouse was developed by Taconic in collaboration with CXR Biosciences. It was created through a targeted replacement of mouse Ahr Exon 3 with a cDNA construct containing human AHR Exons 3-11 in C57BL/6NTac-derived ES cells. This leads to the expression of a fusion protein of amino acids encoded by mouse Exons 1 and 2 and human Exons 3 to 11 driven by the mouse Ahr promoter. The amino acids encoded by Exon 1 and 2 of mouse and human AHR are highly conserved, with only two changes in Exon 1. Targeted ES cells were injected into BALB/cJBomTac blastocysts and the resultant chimeras were backcrossed to a Flpe deleter strain on C57BL/6J to eliminate the selection marker. Taconic received stock in 2009, and the line was embryo transfer derived. The colony was maintained by mating homozygotes.