FGF21 is a hormone with pivotal functions in the regulation of energy homeostasis and metabolism.[17] Chronic FGF21 treatment in diet‐induced obese mice promotes thermogenesis and energy expenditure, resulting in weight loss.[27] Our data demonstrated that the reduced obesity in LKO mice could be attributed to increased energy expenditure. The gene discussed is FGF21; the disease is obesity due to melanocortin 4 receptor deficiency.