PPEF2 and psychiatric disorder: Monoallelic rare coding variants in GRM5, PPEF2 or LRP1B—all related to NMDA receptor (NMDAR; mGlu5) biology—were found in some affected participants in five multiplex nuclear families with schizophrenia [32]; the variants’ absence in unaffected family members was not reported In addition, rare de novo variants in single genes were correlated with psychiatric disorders by many groups [34,35].