GluN1 (GRIN1) forms a tri-heterotetramer with GluN2A (GRIN2A) and GluN2B (GRIN2B). The tetramer includes two molecules of GluN1, one molecule of GluN2A and one molecule of GluN2B (Sheng et al. 1994). The tetrameric structure of the GluN1:GluN2A:GluN2B (GRIN1:GRIN2A:GRIN2B) triheteromeric NMDA receptor was demonstrated on the cryo-EM structure of the Xenopus orthologue (Lu et al. 2017). The majority of native NMDA receptors in adult forebrain are GluN1:GluN2A:GluN2B tri-heteromers, and their pharmacological properties are distinct from the properties of GluN1:GluN2A and GluN1:GluN2B di-heteromers (Hansen et al. 2014).