Plant cytochrome P450s, clan CYP71The number of cytochrome P450s (P450s, CYPs) in plants is considerably larger than in other taxa. In individual plant genomes, CYPs form the third largest family of plant genes; the two largest gene families code for F-box proteins and receptor-like kinases. CYPs have been classified into families and subfamilies based on homology and phylogenetic criteria; family membership is defined as 40% amino acid sequence identity or higher. However, there is a phenomenon called family creep, where a sequence (below 40% identity) is absorbed into a large family; this is seen in the plant CYP71 and CYP89 families. The plant CYPs have also been classified according to clans; land plants have 11 clans that form two groups: single-family clans (CYP51, CYP74, CYP97, CYP710, CYP711, CYP727, CYP746) and multi-family clans (CYP71, CYP72, CYP85, CYP86). The CYP71 clan has expanded dramatically and represents 50% of all plant CYPs; it includes several families including CYP71, CYP73, CYP76, CYP81, CYP82, CYP89, and CYP93, among others. It belongs to the large cytochrome P450 (P450, CYP) superfamily of heme-containing proteins that catalyze a variety of oxidative reactions of a large number of structurally different endogenous and exogenous compounds in organisms from all major domains of life. CYPs bind their diverse ligands in a buried, hydrophobic active site, which is accessed through a substrate access channel formed by two flexible helices and their connecting loop.