In a phase 2 study, add-on therapy with another dAAO inhibitor (TAK-831/luvadaxistat; Neurocrine Biosciences Inc.)was not effective for negative symptoms of schizophrenia but showed a signal for improving cognitive symptoms (Murthy et al., 2021), and a phase 2 study in patients with cognitive impairment associated with schizophrenia (NCT05182476) is underway (Table 1). This evidence concerns the gene DAO and schizophrenia.