FGF21 and obesity due to melanocortin 4 receptor deficiency: While these mice demonstrated both normal food consumption and the protection against diet-induced obesity (Figure 4—figure supplement 1A–C) described above for 2-month-old male mice, they also showed a 30-fold higher level of circulating FGF-21 as compared with control-fed littermates, and this after only 8 weeks on diet (Figure 4—figure supplement 1D).