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).