The Ah receptor mediates the toxicological responses of 2,3,7,8-TCDD and related compounds. Receptor-deficient animals were shown to be resistant to the toxic effects of dioxin, although there is also evidence for the existence of a receptor-independent pathway for dioxin-induced toxicity. In the cytosol the receptor is present in a non-activated ligand binding conformation. Association with Arnt in the nucleus turns the receptor complex into a ligand activated form. The physiological role of the receptor is not yet understood; however, the conservation of the receptor in a wide range of animal species (including humans) suggests a fundamental role in cellular physiology.