On the other hand, in patients with schizophrenia, PSP activity was significantly higher than in controls, especially in male subjects: PSP activity negatively correlated with plasma D-serine and glycine (Gly) concentrations and D-serine/total serine ratio, and positively, albeit weakly, with plasma L-Ser concentration, especially in male patients16. This evidence concerns the gene PSPH and schizophrenia.