FGF21 and obesity due to melanocortin 4 receptor deficiency: On the contrary, KO of Atg7 in insulin target tissues such as skeletal muscle cells or hepatocytes led to the induction of fibroblast growth factor 21 (FGF21) as a “mitokine” due to mitochondrial stress and to the resistance to diet-induced obesity and insulin resistance (Kim K. H. et al., 2013), contrary to the expectation that autophagy deficiency associated with mitochondrial dysfunction in insulin target tissues would lead to insulin resistance.