GluN3B (GRIN3B) binds to GluN1 (GRIN1) to form a di-heteromeric NMDA receptor (Chatterton et al. 2002). The tetrameric structure of the GluN1:GluN3B (GRIN1:GRIN3B) di-heteromer is inferred from the tetrameric architecture of all known glutamate receptors (Traynelis et al. 2010). GluN3B expression gradually increases during development and is expressed at highest levels in the adult motor neurons (reviewed by Paoletti et al. 2013). GluN1:GluN3B (GRIN1:GRIN3B) di-heteromers can be activated by glycine in Xenopus oocytes but not in human embryonic kidney cell line HEK293 (Smothers and Woodward 2007).