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N-methyl-D-aspartate receptors (NMDARs), one of three main classes of ionotropic glutamate receptors, play major roles in synaptic plasticity, synaptogenesis and excitotoxicity.Unlike non-NMDA receptors, NMDARs are thought to comprise a hetero-tetrameric complex mainly composed of NR1 and NR2 subunits.When expressed alone in heterogenous cells, such as HEK293 cells, most of the NR subunits can neither leave the endoplasmic reticulum (ER) nor be expressed in the cell membrane because of the ER retention signals.Only when NMDARs are heteromerically assembled, can the ER retention signals be masked and NMDARs be expressed in the surface membrane.However, the mechanisms underlying NMDAR assembly remain poorly understood.To identify regions in subunits that mediate this assembly, we made a series of truncated or chimeric cDNA constructs.Using fluorescence resonance energy transfer (FRET) measurement in living cells, combined with immunostaining and biochemical analysis, we examined the domain-determining assembly of NMDAR subunits.Our results indicate that the transmembrane region of subunits is necessary and essential for the assembly ofNMDAR subunits, both for the homomer and the heteromer.