AHR is a ligand-activated helix-loop-helix transcription factor involved in the regulation of biological responses to planar aromatic hydrocarbons. AHR has been shown to regulate xenobiotic-metabolizing enzymes such as cytochrome P450. Before ligand binding, AHR is sequestered in the cytoplasm; upon ligand binding, the protein moves to the nucleus and stimulates transcription of target genes. [taken from NCBI Entrez Gene (Gene ID: 196)].