AbstractThe P450 superfamily is a large group of hemoprotein monooxygenases participating in the oxidation of various small molecules. Numerous species of P450 showing various substrate specificities exist in a wide variety of organisms and play significant roles in various metabolic pathways including biosynthesis of biosignal substances and secondary metabolites, degradation of toxic xenobiotics, and adaptive assimilation of unusual carbon sources. The divergence of an ancestral P450 into multiple monooxygenases expanded the ability of living organisms to metabolize small molecules. Most P450 monooxygenases are inducible enzymes, and the factors regulating their expression are as varied as their substrates. Thus, P450 enzymes are versatile hemoproteins not only in their metabolic functions but also in their regulation of gene activation. Such versatility among P450 enzymes is essential for living organisms to adapt to diverse environments.