The sequence and organization of the CYP1A cluster on human chromosome 15 was determined. A human genomic clone from a BAC library, containing both CYP1A1 and CYP1A2 genes, was isolated and sequenced. The results of Southern blot analysis using human genomic DNA were compatible with the structure of the BAC clone. The CYP1A1 and CYP1A2 genes are separated by a 23 kb segment that contains no other open reading frames. The CYP1A1 and CYP1A2 genes are in opposite orientation, revealing that the 5' flanking region is in common between the two genes. Analysis of the sequence obtained revealed the presence of xenobiotic response elements (XREs) previously reported for CYP1A1 and CYP1A2 and several additional consensus sequences for putative XREs. The presence of all the XREs upstream of both genes suggest that some of the regulatory elements known to control CYP1A1 gene expression, could also control CYP1A2 gene expression.