 The ligand-free, cytosolic form of the aryl hydrocarbon receptor is complexed to heat shock protein 90.Binding of ligand, which includes dioxin and polycyclic aromatic hydrocarbons, results in translocation of the ligand-binding subunit only into the nucleus.Induction of enzymes involved in xenobiotic metabolism occurs through binding of the ligand-bound AhR to xenobiotic responsive elements in the promoters of genes for these enzymes.