The melanocortin signaling system plays a role in many physiological outcomes such as skin pigmentation, food intake, sexual behavior and more. Melanocortin receptors (MCRs) 1 through 5, belong to the superfamily of G-protein coupled receptors, and mediate the effects of five different peptides: ACTH, α-MSH, β-MSH and γ-MSH. In addition, this system has endogenous antagonists: agouti and AGRP. Agouti inhibits MCR1 and MCR4, while AGRP antagonizes MCR3 and MCR4. These two antagonists, when overexpressed in mice, give rise to obese animals, and in some cases, obese and diabetic subjects.