KMO and schizophrenia: We examined blood levels of XA and other kynurenine metabolites in a relatively large cohort of patients affected by schizophrenia, taking into account the following aspects: (i) peripheral KYN and 3-HK enter the brain in large amounts, and fuel the kynurenine pathway in the CNS, whereas brain penetration of KYNA, ANA, 3-HANA, and QUINA, is poor1; and (ii) enzymes of the kynurenine pathway are segregated in different cell types in the CNS, with KMO and kynureninase being present in microglia, and KATII in astrocytes32, 33.