The Cytochrome P450 Homepage, maintained by David R. Nelson, is a comprehensive resource for nomenclature and sequence information on cytochrome P450 (CYP) genes. Launched in February 1995, the site has grown to include naming information for 11,512 CYPs. The sequences are manually curated, and the nomenclature system follows an evolutionary scheme to ensure common ancestors among CYP families and subfamilies. The homepage was initially created to provide unlimited space for nomenclature and annotation information, evolving from a simple list of CYP names to a more structured format with sections for nomenclature, databases, BLAST server sequences, 3D structure tours, and species-specific pages. The site includes detailed tables for human and mouse CYPs, with links to various data sources, and offers resources for plant, animal, lower eukaryote, and bacterial CYPs. Future plans include a permanent home at a major sequence repository and the development of deeper time nomenclature to recognize deep-time clades of P450s.The Cytochrome P450 Homepage, maintained by David R. Nelson, is a comprehensive resource for nomenclature and sequence information on cytochrome P450 (CYP) genes. Launched in February 1995, the site has grown to include naming information for 11,512 CYPs. The sequences are manually curated, and the nomenclature system follows an evolutionary scheme to ensure common ancestors among CYP families and subfamilies. The homepage was initially created to provide unlimited space for nomenclature and annotation information, evolving from a simple list of CYP names to a more structured format with sections for nomenclature, databases, BLAST server sequences, 3D structure tours, and species-specific pages. The site includes detailed tables for human and mouse CYPs, with links to various data sources, and offers resources for plant, animal, lower eukaryote, and bacterial CYPs. Future plans include a permanent home at a major sequence repository and the development of deeper time nomenclature to recognize deep-time clades of P450s.