The aryl hydrocarbon receptor (AhR) is a ligand-dependent intracellular transcription factor whose ligands include some of the most infamous xenobiotics, including dioxin (TCDD, 2,3,7,8-tetrachlorodibenzoparadioxin), benzo[a]pyrene, and numerous polyaromatic hydrocarbons from soot particles and coal tar. CAY10464 is a potent and selective AhR antagonist, with a K of 1.4 nM when tested in rabbit liver cytosol preparations. It is inactive as an estrogen receptor ligands even at 100 µM.