The Cytochrome P450 Homepage is a comprehensive resource for nomenclature and sequence information on cytochrome P450 (CYP) genes. Established in 1995, it has continuously provided naming information for over 11,512 CYPs. The site's nomenclature system is based on evolutionary relationships, grouping CYP families and subfamilies by common ancestry. The homepage is organized into sections for animals, plants, lower eukaryotes, and bacteria, with detailed information on each species' CYPs. It includes sequence alignments, phylogenetic trees, and links to databases. The homepage also features a P450 BLAST server for sequence searches and a 3D structure tour of P450 proteins. The site has a dedicated section for fungal CYPs and includes data from numerous species, including 55 complete fungal genomes. The homepage has been updated with new data and includes links to publications, talks, and species-specific information. The site is maintained by David R. Nelson, who manually curates sequences and updates the nomenclature system. The homepage has faced challenges in keeping up with the rapid expansion of CYP sequences, but it remains a vital resource for researchers. The site also includes information on P450s in non-traditional organisms, such as Mimivirus, and has links to other P450-related databases. The homepage is primarily a nomenclature resource, but it also serves as a platform for gene annotation and sequence analysis. The site has been visited by many researchers and has been updated regularly. Future plans include expanding the nomenclature system and securing funding for continued annotation efforts. The homepage is a critical resource for understanding the diversity and evolution of CYP genes across different species.The Cytochrome P450 Homepage is a comprehensive resource for nomenclature and sequence information on cytochrome P450 (CYP) genes. Established in 1995, it has continuously provided naming information for over 11,512 CYPs. The site's nomenclature system is based on evolutionary relationships, grouping CYP families and subfamilies by common ancestry. The homepage is organized into sections for animals, plants, lower eukaryotes, and bacteria, with detailed information on each species' CYPs. It includes sequence alignments, phylogenetic trees, and links to databases. The homepage also features a P450 BLAST server for sequence searches and a 3D structure tour of P450 proteins. The site has a dedicated section for fungal CYPs and includes data from numerous species, including 55 complete fungal genomes. The homepage has been updated with new data and includes links to publications, talks, and species-specific information. The site is maintained by David R. Nelson, who manually curates sequences and updates the nomenclature system. The homepage has faced challenges in keeping up with the rapid expansion of CYP sequences, but it remains a vital resource for researchers. The site also includes information on P450s in non-traditional organisms, such as Mimivirus, and has links to other P450-related databases. The homepage is primarily a nomenclature resource, but it also serves as a platform for gene annotation and sequence analysis. The site has been visited by many researchers and has been updated regularly. Future plans include expanding the nomenclature system and securing funding for continued annotation efforts. The homepage is a critical resource for understanding the diversity and evolution of CYP genes across different species.