The pore of the NMDA receptor is blocked by binding of Mg2+ ions. The strength of the block depends on the subunit composition of the NMDA receptor, as both the GluN1 (GRIN1, NR1) subunit and GluN2 (GRIN2, NR2) subunits interact with Mg2+ (Nowak et al. 1984, Mayer et al. 1984, Wollmuth et al. 1998, Vargas-Caballero and Robinson 2004, Siegler Retchless et al. 2012, Schwartz et al. 2012, Clarke et al. 2013, reviewed by Iacobucci and Popescu 2017).