Exposure to halogenated and polycyclic aromatic hydrocarbons results in a wide range of toxic and carcinogenic responses. Most of these exposure effects are mediated by the aryl hydrocarbon receptor (AhR). AHR is a cytosolic protein associated with chaperone and immunophilin-like proteins (HSP90, XAP2, TEBP). Upon ligand activation, AHR dissociates from the complex, translocates into the nucleus, dimerizes with the aryl hydrocarbon nuclear translocator (ARNT), recruits several classes of co-activators, and binds to the xenobiotic response element (XRE). This AhR complex induces transcriptional activation of genes encoding xenobiotic metabolizing enzymes (CYP1A1, CYP1A2, CYP1B1), phase II metabolizing enzymes (GST, NADPH-quinone oxidoreductase, UDP-glucuronyl-S-transferase) and other growth factors and proteins, involved in cell cycle progression (p27Kip1, p21Cip1) and apoptosis (Bax, Fas, Fasl)...