The glucagon receptor (GCGR) is a Class B GPCR that has an important role in maintenance of glucose homeostasis and, as such, is considered to be a valuable target for the treatment of diabetes. Mutations of the GCGR gene are associated with congenital noninsulin-dependent diabetes, and inhibition of GCGR in vivo lowers blood glucose and improves glucose tolerance in obese diabetic mice. Studies of immunologic blockade of GCGR date back to at least 1996 when polyclonal antibodies were generated to identify important epitopes to target for receptor antagonism.