DAO and schizophrenia: The glycine modulatory binding site on the NMDA receptor is considered a more promising target, and direct or indirect glycine modulatory site modulators, including glycine, D-cycloserine, D-serine, glycine transporter 1 (GlyT1) inhibitors, and D-amino acid oxidase (DAAO) inhibitors, have been actively researched in clinical trials and, as discussed in more detail below, show promise in the treatment of schizophrenia following the glutamatergic hypothesis of schizophrenia [48].