AHR and obesity due to melanocortin 4 receptor deficiency: AHR signaling and obesity. There are four well-characterized Ahr allelic variants in mice, of which there is a 10-fold difference in the affinity for the AHR ligand between the most-responsive allele (Ahrb1 in the B6 mouse) and least-responsive allele (Ahrd in the B6.D2 mouse) (Poland et al. 1994).