CYP1A2 is one of the major hepatic cytochrome P450s that is involved in the metabolism of many drugs, as well as in the activation of chemical carcinogens. To elucidate the transcriptional regulation of the constitutive expression of the mouse Cypla2 gene, the 4.8-kbp 5(')-flanking region of the gene was analyzed for transcriptional activity using a primary cultured mouse hepatocyte system. With 5(')- and 3(')-deletion analysis, two enhancer elements, i.e., a 20-bp DNA fragment (E1) from -4401 to -4382 and a 9-bp (E2) from -4300 to -4292, were identified. E1 and E2 contain a phorbol 12-O-tetradecanoate-13-acetate (TPA)-responsive element (TRE) and TRE-like element, respectively. Electrophoretic mobility shift assay confirmed specific binding between these two enhancer elements and nuclear proteins. Site-directed mutagenesis assay suggested that the TRE element in E1 is essential for constitutive expression of the mouse Cypla2 gene.