GluN1 (GRIN1) forms a di-heterotetramer with GluN2D (GRIN2D) in heterologous expression systems, although stoichiometry is unclear in neurons. The tetramer includes two molecules of GluN1 and two molecules of GluN2D (Dunah et al. 1998). GluN2D is expressed in the brain during embryonic development. In adult brain, GluN2D expression is low and mostly restricted to diencephalon and mesencephalon, as well as GABAergic interneurons (von Engelhardt et al. 2015, Perszyk et al. 2016, reviewed by Paoletti et al. 2013). The ligand binding domain of GluN2D interacts with agonists in a similar fashion to other GluN2 subunits (Vance et al. 2011, Hansen et al. 2013).