cytochrome P450 family/clan 97CYPs 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. 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). Members of the CYP97 clan include Arabidopsis thaliana cytochrome P450s 97A3 (CYP97A3), CYP97B3, and CYP97C1. CYP97A3 is also called protein LUTEIN DEFICIENT 5 (LUT5) and CYP97C1 is also called carotene epsilon-monooxygenase or protein LUTEIN DEFICIENT 1 (LUT1). These cytochromes function as beta- and epsilon-ring carotenoid hydroxylases and are involved in the biosynthesis of xanthophylls. CYP97 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.