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. This evidence concerns the gene KYNU and schizophrenia.