UCP1 and obesity due to melanocortin 4 receptor deficiency: β3-Adrenoceptor-deficient mice show a mild obesity phenotype with modestly increased fat stores [89], and mice lacking the three known β-adrenoceptor subtypes are more obese on a standard chow diet and even more severe on a high-fat diet, show lower basal metabolic rate, and lack cold-induced thermogenesis and UCP1 increase in BAT, leading to hypothermia [2].