The Ah (dioxin) receptor nuclear translocator protein (Arnt) belongs to the basichelix-loop-helix (bHLH) family of transcription factors and is required forAh receptor function. The Ah receptor binds, and mediates the carcinogeniceffects of a variety of environmental pollutants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin and polycyclic aromatic hydrocarbons. Arnt is a structural component of theXRE-binding form of the Ah receptor. These proteins are class VII members of the basic helix-loop-helix (bHLH) family. The bHLH domain may be responsible for interacting with both the XRE and the ligand-bindingsubunit [<cite idref="PUB00003697"/>, <cite idref="PUB00005150"/>].<p>The activated Ah receptor and Arnt protein bind DNA as a heterodimer. Both proteins contain PAS homology regions, which in Drosophila PER and SIM proteins function as dimerisation domains [<cite idref="PUB00003697"/>].</p>