The melanocortin receptor 3 (MC3 receptor) is Gs-coupled GPCRs expressed in brain, placental, and gut tissues but not in melanoma cells or in the adrenal gland. MC3 receptor knockouts exhibit a metabolic syndrome. At 4-6 months old, MC3 receptor knockout mice show increased fat mass, reduced lean mass and a higher feed efficiency, with normal metabolic rates. MC3 knockout mice are hyperleptinaemic and males are often mildly hyperinsulinaemic. GenScript's cloned human MC3R-expressing cell line is made in the 293/CRE-Luciferase host.