A targeting vector was designed to insert the 6 kb long human chimeric cDNA clone GRIN2A/GRIN2B in-frame, leaving the 34 amino-acid long signal peptide sequence intact. Amino acid residues 35-282 of the human glutamate receptor, ionotropic, NMDA2A (epsilon 1) and the carboxyl terminal remainder of the human glutamate receptor, ionotropic, NMDA2B (epsilon 2) residues were inserted into the mouse locus. The vector was inserted into exon 4 (the first coding exon) of the endogenous locus.