DAO (D-amino acid oxidase) is a flavoenzyme that catalyzes the oxidative deamination of D-amino acids with broad substrate specificity 1. The enzyme plays multiple critical roles: it catabolizes endogenously synthesized D-amino acids, those from gastrointestinal bacteria, and dietary sources 2. In the brain, DAO regulates D-serine levels, a co-agonist of NMDA receptors that modulates synaptic transmission 3. The enzyme also catalyzes initial steps in dopamine biosynthesis and chiral inversion of nitric oxide synthase inhibitors 3. Additionally, hydrogen peroxide generated during DAO catalysis contributes to antimicrobial defense by enhancing oxidative killing in phagocytic leukocytes and protecting against intestinal bacterial colonization 2. DAO deficiency is implicated in histamine intolerance (HIT), a non-immunological disorder characterized by impaired histamine metabolism 2. HIT manifests as gastrointestinal and systemic symptoms; diagnosis relies primarily on clinical assessment and response to low-histamine diets, as serum DAO measurements show limited diagnostic utility 4. Genetic associations link DAO polymorphisms to schizophrenia risk, suggesting dysregulation of glutamatergic neurotransmission 5. Notably, DAO appears a master gene in schizophrenia pathogenesis, interacting with multiple susceptibility loci including DISC1, NRG1, and RASD2 6. These findings position DAO at the intersection of neurotransmitter homeostasis, immune function, and digestive health.