The sequence and organization of theCYP1Acluster on human chromosome 15 was determined. A human genomic clone from a BAC library, containing bothCYP1A1andCYP1A2genes, was isolated and sequenced. The results of Southern blot analysis using human genomic DNA were compatible with the structure of the BAC clone. TheCYP1A1andCYP1A2genes are separated by a 23 kb segment that contains no other open reading frames. TheCYP1A1andCYP1A2genes 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 forCYP1A1andCYP1A2and 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 controlCYP1A1gene expression, could also controlCYP1A2gene expression.