Flavin is a tricyclic heteronuclear organic ring whose biochemical source is the vitamin riboflavin. The flavin moiety is often conjugated with an adenosine diphosphate to form flavin adenine dinucleotide (FAD), and in other circumstances, is found as flavin mononucleotide (or FMN), a phosphorylated form of riboflavin. The flavin group is capable of undergoing oxidation-reduction reactions, and can accept either one electron in a two step process or can accept two electrons at once. In the form of FADH2, it is one of the cofactors that can transfer electrons to the electron transfer chain.
Flavin adenine dinucleotide is a cofactor in the enzymes D-amino acid oxidase, glucose oxidase, and xanthine oxidase.