The Aryl Hydrocarbon Receptor (AHR), also known as the dioxin receptor, is a ligand-activated helix/loop/helix transcription factor found in a variety of vertebrate species. The known ligands for AHR are foreign planar aromatic compounds, such as polycyclic aromatic compounds and halogenated aromatic compounds such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).Unlike the steroid/thyroid hormone receptors, there is no known physiological ligand for the AH Receptor. Studies indicate that in non-ligand activated cells, AHR is found complexed with HSP90 predominantly in the cytoplasm. Upon binding to an agonist, the ligand-activated AHR is believed to transform to a nuclear, DNA binding form. This transformation process appears to involve dissociation of HSP90 from AHR followed by formation of a heterodimer with AHR nuclear translocator protein (Arnt). The AHR-ligand complex appears to initiate gene transcription of cytochrome P450 1A1.