Molecular model of an aryl hydrocarbon receptor (AHR, pink surface) binding to DNA (deoxyribonucleic acid) along with aryl hydrocarbon receptor nuclear translocator (purple surface). The area on the DNA which AHR binds to is called the dioxin response element (DRE), which is recognised by DNA-binding arginine residues. The specific DNA-binding residues discriminates the DRE from the closely related hypoxia response element (HRE). The aryl hydrocarbon receptor belongs to the PAS (PER-ARNT-SIM) family of transcription factors which mediate physiological responses to numerous environmental pollutants and cellular metabolites.