Karakas and Furukawa5 and Lee et al.6 independently report the first X-ray structures of an NMDA receptor — the GluN1–GluN2B receptor. The large cytoplasmic domain is absent from both structures. This figure is based on Lee and colleagues' structure. a, In this surface representation of the receptor, the two GluN1 subunits are in different shades of blue, and the two GluN2B subunits are in different shades of red. In the membrane region, the GluN1 and GluN2B subunits alternate around a closed central pore (inset). b, The receptor displays a layered architecture with a transmembrane domain (TMD; green) that forms an ion channel, four ligand-binding domains (LBDs; orange) and four amino-terminal domains (ATDs; purple). Short linking segments (cyan) connect the ATDs to the LBDs, and the LBDs to the TMD. A modulator and agonist are shown in black. The binding site for pore blockers is indicated.