FGF21 and obesity due to melanocortin 4 receptor deficiency: This broader definition was first applied to fibroblast growth factor 21 (FGF21), which was found to be induced in mouse models of autophagy deficiency by targeted disruption of Atg7 (autophagy-related 7) in skeletal muscles or the liver, leading to a protection from obesity and insulin resistance of these mice [13].