We provide here a list of 154 P450 genes and seven putative pseudogenes that have been characterized as of October 20, 1990. These genes have been described in a total of 23 eukaryotes (including nine mammalian and one plant species) and six prokaryotes. Of 27 gene families so far described, 10 exist in all mammals. These 10 families comprise 18 subfamilies, of which 16 and 14 have been mapped in the human and mouse genomes, respectively; to date, each subfamily appears to represent a cluster of tightly linked genes. We propose here a modest revision of the initially proposed (Nebert et al., DNA 6, 1-11, 1987) and updated (Nebert et al., DNA 8, 1-13, 1989) nomenclature system based on evolution of the superfamily. For the gene we recommend that the italicized root symbol CYP for human (Cyp for mouse), representing cytochrome P450, 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 should precede the final number in mouse genes. We suggest that the human nomenclature system be used for other species. This system is consistent with our earlier proposed nomenclature for P450 of all eukaryotes and prokaryotes, except that we are discouraging the future use of cumbersome Roman numerals.