AH receptor-interacting protein (AIP) is a 37 kD immunophilin-like factor found in a variety of tissues with expression levels ranging from high (spleen, thymus, pituitary heart, placenta and skeletal muscle) to low (liver, kidney and lung). It mediates aryl hydrocarbon receptor (AhR) signaling either through ligand receptivity and/or modulating nuclear targeting and has been shown to bind to both the AhR itself as well as the AhR nuclear translocator (ARNT).