Aspects of cytochrome P450 (CYP) nomenclature are addressed. The rules for naming a P450 are outlined, though individuals should not name their own genes. The nomenclature is presented as a unifying principle to enhance communication across disciplines. Because of the historical nature of gene sequencing, sometimes names have to be changed, but this is kept to a bare minimum to avoid confusion in the literature. CYP names have now reached four digits owing to proliferation of CYP families in the fungi and lower eukaryotes. For example, CYP5034A1 is from Ustilago maydis. P450 sequence motifs are described that are useful in making global alignments. CYP clans are defined as clusters of CYP families. The clan names are useful in describing higher-order evolution of the gene superfamily. The nomenclature of orthologs and pseudogenes is also discussed.