NMDA receptors (NMDARs) are a class of ionotropic glutamate receptors (iGluRs) that are essential for neuronal development, synaptic plasticity, learning and cell survival. Several features distinguish NMDARs from other iGluRs and underlie the crucial roles NMDARs play in nervous system physiology. NMDARs display slow deactivation kinetics, are highly Ca2+ permeable, and require depolarization to relieve channel block by external Mg2+ ions, thereby making them effective coincidence detectors. These properties and others differ among NMDAR subtypes, which are defined by the subunits that compose the receptor. NMDARs, which are heterotetrameric, commonly are composed of two GluN1 subunits and two GluN2 subunits, of which there are four types, GluN2A–D. ‘Diheteromeric’ NMDARs contain two identical GluN2 subunits. Gating and ligand-binding properties (e.g. deactivation kinetics) and channel properties (e.g. channel block by Mg2+) depend strongly on the GluN2 subunit contained in diheteromeric NMDARs. Recent work shows that two distinct regions of GluN2 subunits control most diheteromeric NMDAR subtype-dependent properties: the N-terminal domain is responsible for most subtype dependence of gating and ligand-binding properties; a single residue difference between GluN2 subunits at a site termed the GluN2 S/L site is responsible for most subtype dependence of channel properties. Thus, two structurally and functionally distinct regions underlie the majority of subtype dependence of NMDAR properties. This topical review highlights recent studies of recombinant diheteromeric NMDARs that uncovered the involvement of the N-terminal domain and of the GluN2 S/L site in the subtype dependence of NMDAR properties.