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Amino acids can exist in two forms called stereoisomers, L- and D-isomers. All common amino acids except glycine have L- and D-enantiomers. Although L-amino acids were previously thought to be the only natural form, D-amino acids, including D-serine, have been found in biological organisms. D-serine is a co-agonist of N-methyl-D-aspartate (NMDA) receptors and is synthesized and degraded by specific enzymes. The distribution of D-serine and NMDA receptors suggests that D-serine acts as an endogenous coagonist on the glycine modulatory site of NMDA receptor subunits. Selective degradation of D-serine with an enzyme called D-amino acid oxidase (DAAO) attenuated NMDA receptor function, confirming a role for endogenous D-serine in synaptic transmission. It is proposed that the synaptic NMDA receptor is activated by D-serine, while the extrasynaptic NMDA receptor is gated by glycine.