We have sequenced the human CYP1A2 (cytochrome P(3)450) gene, 1,906 basepairs (bp) of the 5' flanking region, and 113 bp of the 3' flanking region. The gene spans almost 7.8 kilobases, comprising seven exons and six introns. The transcriptional start site was determined by both primer extension and S1 mapping. Including the first noncoding exon of 55 bp, the entire mRNA is 3,121 bp in length, and the open reading frame, starting with nucleotide 10 of exon 2, encodes 515 amino acids (mol wt = 58,294). Between the human CYP1A2 and CYP1A1 (cytochrome P(1)450) genes, exons 2, 4, 6, and especially 5 are strikingly conserved in both nucleotide similarity and total number of bases. Alignment of the upstream sequences and exon 1 of human CYP1A2 with that of mouse or rat CYP1A2 revealed two possibly significant regions of similarity: 1) 68% in the approximately 150 bases immediately 5' from the mRNA cap site and 2) 80% identify between the human -841 to -758 segment and the mouse -1,529 to -1,439 segment. The canonical 5-bp box (CACGC), found upstream of all mammalian CYP1A1 genes to date and believed to interact with the inducer.aromatic hydrocarbon receptor complex, was not found on either strand in the 1,906 bp of the 5' flanking region of human CYP1A2. In contrast, alignment of the upstream sequences, exon 1, and intron 1 of human CYP1A1 with that of mouse or rat CYP1A1 revealed large, highly conserved regions. Conserved regions were found in intron 1 of the human, mouse, and rat CYP1A2 gene. These data suggest that the regulatory elements controlling the CYP1A2 gene might differ in location from those controlling the CYP1A1 gene. Among 12 human liver samples, striking differences (greater than 15-fold) in the 3.3-kilobase 1A2 mRNA levels were seen. This result may reflect significant genetic differences in constitutive and/or inducible CYP1A2 gene expression that could play an important role in individual risk of environmental toxicity or cancer.