Cytochrome P450 genes from the CYP2A, CYP2B, and CYP2F subfamilies form a tight cluster which we have localized on the detailed physical map of human chromosome 19. The corresponding three gene subfamilies are also clustered in the mouse genome, on the region of chromosome 7 known to be syntenic to human chromosome 19. One hundred eight cosmid clones from the human P450 region were assembled into a single contig of 350 kb, restriction mapped, and probed with cDNAs from the three gene subfamilies. A total of 11 genes were identified in humans, including five from the 2A subfamily, three from the 2B subfamily, and three from the 2F subfamily; at least six of the 11 are pseudogenes. The organization of the genes, with members of the three subfamilies intermixed, indicates that the evolution of this gene cluster has been complex. The modern gene arrangement in humans is probably the result of a series of tandem duplications, plus at least one inverted duplication. The identification of all genes and pseudogenes in this cluster also makes it possible to determine the origins of some previously known variant P450 transcripts.