FMOs are one subfamily of class B external flavoprotein monooxygenases (EC 1.14.13), which belong to the family of monooxygenase oxidoreductases, along with the other subfamilies Baeyer-Villiger monooxygenases and microbial N-hydroxylating monooxygenases. FMO's are found in fungi, yeast, plants, mammals, and bacteria. Plant FMO's play a role in defending against pathogens and catalyze specific steps in the biosynthesis of auxin, a plant hormone. Plant FMO's also play a role in the metabolism of glucosinolates. These non-xenobiotic roles of plant FMO's suggest that other FMO functions could be identified in non-plant organisms.