AHR (Aryl-hydrocarbon receptor; also bHLHE76) is a 110 kDa member of the bHLH/PAS transcription factor family. It is widely expressed (breast, lung, liver), and serves many functions. First, it binds multiple xenobiotic chemicals in the cytoplasm. This induces dimerization with ARNT, translocation to the nucleus, and activation of P450 genes such as CYP1A1 and UGT1A6. Second, it appears to block cell cycle progression, possibly via a down-regulation of CDK proteins. And third, it blocks apoptosis by interacting with E2F1, thus silencing TP73 and Apaf1 genes. Human AHR is 848 amino acids (aa) in length. It contains a 10 aa prosegment, plus a 838 aa mature molecule that contains a DNA binding motif (aa 13-40), a bHLH region (aa 41-81), and two PAS domains (aa 111-342). Over aa 704-848, human AHR shares 70% aa identity with mouse AHR.