GluN1 (GRIN1) is assumed to form a tri-heterotetramer with GluN2A (GRIN2A) and GluN2D (GRIN2D). The tetramer includes two molecules of GluN1, one molecule of GluN2A and one molecule of GluN2D (Dunah et al. 1998). The tetrameric structure of the GluN1:GluN2A:GluN2D (GRIN1:GRIN2A:GRIN2D) tri-heterotetramer is assumed to follow from the cryo-EM structure of the triheteromeric Xenopus GluN1:GluN2A:GluN2B NMDA receptor (Lu et al. 2017).