GluN3A (GRIN3A) interacts with GluN1 (GRIN1) to form a di-heteromeric NMDA receptor (Perez-Otano et al. 2001, Chatterton et al. 2002). The tetrameric structure of the GluN1:GluN3A (GRIN1:GRIN3A) di-heteromer is inferred from the tetrameric architecture of all known glutamate receptors (Traynelis et al. 2010). GluN3A expression reaches the highest level in early postnatal period and then declines (reviewed by Paoletti et al. 2013). GluN1:GluN3A di-heteromers can be activated by glycine in Xenopus oocytes (Smothers and Woodward 2007), and the action of glycine can be greatly enhanced by diminishing the ability of GluN1 subunit to interact with glycine (Awobuluyi et al. 2007, Kvist et al. 2013, Grand et al. 2018).