In rat brain, 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 was demonstrated for the Xenopus GluN1:GluN2A:GluN2B NMDA receptor (Lu et al. 2017).