The Ahr Knockout mouse was developed by Taconic in collaboration with CXR Biosciences. It was generated by crossing an AHR humanized mouse line (Model 9165) with a Cre deleter mouse. The Humanized AHR Mouse model 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. 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 selection markers. The Ahr Knockout Mouse model was derived from the Humanized AHR Mouse line through a Cre-mediated deletion of the human AHR sequence. Mouse exon 3 is deleted in the knockout, resulting in an out of frame splicing of exons 2 to exon 4. The absence of functional Ahr protein was proven experimentally. Taconic received stock in 2009, and the line was embryo transfer derived. The colony was maintained by mating homozygotes.