GRIN2B and epilepsy: NMDA receptors are tetrameric ligand-gated ion channels permeable to Na+,K+, and Ca2+, composed of 2 glycine-binding NR1 subunits and 2glutamate-binding NR2 subunits (NR2A, NR2B, NR2C, NR2D).25,26 Subunit composition of NMDA receptors isspatially and temporally regulated with a switch from predominant NR2B expression in earlydevelopment to more prominent synaptically localized NR2A expression at later stages,27,28 which might explainthe tendency toward earlier onset epilepsy phenotypes in GRIN2B versusGRIN2A mutation carriers.