We provide here a list of 481 P450 genes and 22 pseudogenes, plus all accession numbers that have been reported as of October 18,1995. These genes have been described in 85 eukaryote (including vertebrates, invertebrates, fungi, and plants) and 20 prokaryote species. Of 74 gene families so far described, 14 families exist in all mammals examined to date. These 14 families comprise 26 mammalian subfamilies, of which 20 and 15 have been mapped in the human genome and the mouse genome, respectively. Each subfamily usually represents a cluster of tightly linked genes widely scattered throughout the genome, but there are exceptions. Interestingly, theCYP51family has been found in mammals, filamentous fungi and yeast, and plants - attesting to the fact that this P450 gene family is very ancient. One functionalCYP51gene and two processed pseudogenes, which are the first examples of intronless pseudogenes within the P450 superfamily, have been mapped to three different human chromosomesThis revision supersedes the four previous updates in which a nomenclature system, based on divergent evolution of the superfamily, has been described. For the gene, we recommend that the italicized root symbol'CYP'for human ('Cyp' for mouse and Drosophila), representing 'cytochromeP450' be followed by an Arabic number denoting the family, a letter designating the subfamily (when two or more exist), and an Arabic numeral representing the individual gene within the subfamily. A hyphen is no longer recommended in mouse gene nomenclature.'P'('ps' in mouse and Drosophila) after the gene number denotes a pseudogene;'X'after the gene number means its use has been discontinued. If a gene is the sole member of a family, the subfamily letter and gene number would be helpful but need not be included. The human nomenclature system should be used for all species other than mouse and Drosophila. The cDNAs, mRNAs and enzymes in all species (including mouse) should include all capital letters, and without italics or hyphens. This nomenclature system is similar to that proposed in our previous updates