Xenopus laevis oocytes have contributed greatly to the study of glutamate receptors. The isolation of the first cDNA clone for a glutamate receptor channel, GluR1, was achieved by utilizing an oocyte expression cloning system (1 ). In 1991–1992, three groups reported the isolation of cDNA clones encoding of N -methyl-d -aspartate (NMDA) receptor channel subunit (2 –4 ). Two of the three groups employed the oocyte system to isolate NMDA receptor clones (2 ,3 ). Furthermore, a metabotropic glutamate receptor (mGluR) clone was also identified with the oocyte system (5 ).