This addendum was compiled by the authors to clarify some points presented in the review article titled “Expression, Function and Regulation of Mouse Cytochrome P450 Enzymes: Comparison With Human Cytochrome P450 Enzymes”, which appeared in Current Drug Metabolism (2009) 10 (10), 1151-1183. Table 1 lists Cyp2c22, Cyp2c23, Cyp2c51 and Cyp2b20 as mouse P450 genes. Mouse Cyp2c22 and Cyp2c23 genes (ref. 440) should not be included in Table 1 because they are believed to be the same as Cyp2c70 and Cyp2c44, respectively (see mouse Master Table in Dr. Nelsons P450 homepage, http://drnelson.uthsc.edu/mouse.table.html). The mouse Cyp2c51 gene (ref. 119) is probably a combination of mouse Cyp2c52-ps and Cyp2c69 genes and may not be a separate gene (see Dr. Nelsons P450 homepage). The mouse Cyp2b20 gene (Swiss-Prot accession number Q62397) may be identical with Cyp2b10, although recent studies (refs. 9, 60 and 180) have suggested that Cyp2b20 and Cyp2b10 may be different genes. This matter remains to be resolved. If the following mouse P450 genes, Cyp2c22, Cyp2c23, Cyp2c51 and Cyp2b20, are removed from Table 1 and the Cyp2ab1 gene (which is not a pseudogene, ref. 8) is added, then the total mouse P450 gene count would be 102 genes, as originally reported in ref. 8, and not 105 genes as stated on page 1151. With regard to the human cytochrome P450 data presented in Table 2, CYP4F3A and CYP4F3B enzymes were generated by alternative splicing of the CYP4F3 gene that encodes the CYP4F3 enzyme (ref. 174). Thus, CYP4F3A and CYP4F3B enzymes are not encoded by separate cytochrome P450 genes. It thus appears that there are 57 human P450 genes, as originally reported in ref. 8, and not 58 genes as stated on page 1151. In addition to the human P450 enzymes listed in Table 2, additional human P450 enzymes have been discovered as a result of gene splicing. The authors are indebted to Drs. Daniel Nebert and David Nelson for their expert advice on the subject matter presented in this addendum.