Immunoblots from Xenopus oocytes expressing either wt or mutant subunits. M1 indicates the GluN1 monomer (~110 kDa); M2 the GluN2B monomer (~180 kDa); D1/1 the GluN1 homodimer (~220 kDa); and D1/2 the GluN1/GluN2B heterodimer (~290 kDa). Lower panels: immunoblots in reducing conditions (+ β-mercaptoethanol). N.I., non-injected oocytes 