A third target currently under investigation, D-amino acid oxidase (DAAO) (10), which also indirectly affects occupancy of the NMDAR co-agonist site, is also based on the “glutamate hypothesis.” A central pillar of this hypothesis is that hypofunction of NMDARs causes some or all of the symptoms of schizophrenia (11–13). Here, DAO is linked to schizophrenia.