The aryl hydrocarbon receptor (AhR or dioxin receptor) is a ligand‐activated transcription factor that is considered to mediate pleiotropic biological responses such as teratogenesis, tumour promotion, epithelial hyperplasia and the induction of drug‐metabolizing enzymes to environmental contaminants usually represented by 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD). In contrast to the role of AhR in the regulatory mechanism of xenobiotic‐metabolizing enzymes, there is no direct proof that the AhR is involved in the teratogenic effects of TCDD.

To gain insight into the physiological and teratogenic role of the AhR, we have used gene targeting in mice to disrupt the murine Ahr gene by homologous recombination. Ahr‐null mice were viable and fertile and were apparently normal at birth, but displayed a slightly slower growth rate than wild‐type mice for the first few weeks of life. When pregnant dams were administered with TCDD by gavage, at a dose of 40 μg/kg body weight at gestation day 12.5, none of the Ahr‐null mutant foetuses were sensitive to the teratogenic effects of TCDD, although almost all wild‐type foetuses suffered from cleft palate and hydronephrosis. In heterozygous Ahr^+/− genotypes, nearly all foetuses suffered from hydronephrosis in response to TCDD treatment, while haplo‐insufficiency was observed in the incidence of cleft palate.

These results clearly show that the AhR is involved in the malformation of the palate and kidney in mouse embryos caused by TCDD and suggests that the mechanism of its involvement differs between the cleft palate and hydronephrosis.