Flavin is a general name given to molecules having the heteroaromatic ring system of 7,8-dimethylisoalloxazine but practically means riboflavin (Rfl), flavin adenine dinucleotide (FAD), and flavin mononucleotide (FMN) in biological systems, whose structures are illustrated in Fig. 1, together with the atomic numbering scheme and ring numbering of the isoalloxazine moiety. As the isoalloxazine skeleton cannot be synthesized in human cells, it is obtained from diet as Rfl (vitamin B-2). FAD and FMN can act as cofactors in flavoenzymes but Rfl does not. Most flavoenzymes catalyze redox reactions of substrates (Miura, Chem Rec 1:183-194, 2001). When O-2 serves as the oxidant in the oxidation half cycle of an enzymic reaction, the enzyme is called "flavo-oxidase" but when others do, the enzyme is called "flavo-dehydrogenase." The difference between the two types of oxidative catalysis arises from delicate differences in the pi-electron distributions in the isoalloxazine ring, which can be revealed by Raman spectroscopy (Miura, Chem Rec 1:183-194, 2001). Since a flavin is an extremely versatile molecule, the scientific field including chemistry, biochemistry, and enzymology is collectively called "flavonology." It was found recently, however, that the flavin also acts as a chromophore to initiate light-induced DNA repair and signal transductions (Sancar, Chem Rev 103:2203-2237, 2003).