FGF21 administration has potentbeneficial effects on obesity and diabetes in humans, monkeys,and rodents (Kharitonenkov, Adams, 2013), it reduces bodyweight, increases insulin sensitivity, normalizes blood glucoselevels (Kharitonenkov et al., 2005; Coskun et al., 2008; Xuet al., 2009). Here, FGF21 is linked to obesity due to melanocortin 4 receptor deficiency.