GRIN2A and phenylketonuria: Although the effects of L-phenylalanine at the concentration (0.1–0.4 mM) observed in the brains of patients with phenylketonuria on NMDAR activity are unknown, the plasma concentration (> 1 mM) of L-phenylalanine in patients with phenylketonuria inhibits NMDARs, and chronic exposure to high concentrations of L-phenylalanine upregulates the density of NMDARs in Pahenu2 brains with enhanced expression of GluN1 and GluN2A subunits and decreased GluN2B expression33,37,38.