In rat brain, GluN2B (Grin2b) forms a di-heterotetramer with GluN1 (Grin1). The tetramer includes two molecules of GluN1 and two molecules of GluN2B (Sheng et al. 1994, Karakas and Furukawa 2014). GluN2B expression starts during embryonic development and continues in adult brain (Sheng et al. 1994). Recombinant rat GluN1 and GluN2B form functional NMDA receptors when expressed in Xenopus oocytes (Monyer et al. 1992).