Kynurenine 3-monooxygenase (KMO) is an enzyme that catalyzes the hydroxylation of kynurenine to 3-hydroxykynurenine, which is the second step in the kynurenine pathway of tryptophan metabolism. The kynurenine pathway is a major route for the degradation of tryptophan, an essential amino acid that cannot be synthesized by the human body. KMO is expressed in a variety of tissues, including the liver, brain, and immune cells.