G-protein-coupled receptor oligomerisation is a widespread phenomenon. One of the best-studied examples is the metabotropic GABA receptor. This so-called constitutive receptor is formed by heterodimerization of GABAR1 and GABAR2 subunits. Expression of the GABAR1 without the GABAR2 in heterologous systems leads to retention of the subunit in the endoplasmic reticulum. Expression of the GABAR2 subunit alone, meanwhile, leads to surface expression of the subunit, although with no functional activity (i.e., the receptor does not bind agonist and cannot initiate a response following exposure to agonist). Expression of the two subunits together leads to plasma membrane expression of functional receptor. It has been shown that GABAR2 binding to GABAR1 causes masking of a retention signal of functional receptors.