GCG and obesity due to melanocortin 4 receptor deficiency: However, currently the only GPCR agonists approved for obesity treatment are targeting incretin receptors (receptors for glucagon-like peptide-1 (GLP1R), glucose-dependent insulinotropic peptide (GIP), glucagon (GCGR)) (Fig. 1).2 Although the melanocortin receptor MC4R seems to be a logical therapeutic target as well, the use of agonists is limited due to cardiovascular side effects.