Family C GPCRs have a large extracellular N-terminus which binds the orthosteric (endogenous) ligand. The shape of this domain is often likened to a clam. Several allosteric ligands to these receptors have been identified and these bind within the seven transmembrane region.
^Bräuner-Osborne H, Wellendorph P, Jensen AA (2007). "Structure, pharmacology and therapeutic prospects of family C G-protein coupled receptors". Curr Drug Targets8 (1): 169–84. doi:10.2174/138945007779315614. PMID17266540.