A CSF and postmortem brain study also revealed a 25% decreases in D-serine levels and the D/L-serine ratio in the CSF of schizophrenia patients, suggesting that reduced brain SR and elevated D-amino acid oxidase (DAO) protein levels may contribute to the lower D-serine levels observed in the CSF of schizophrenic patients (28). This evidence concerns the gene DAO and schizophrenia.