A growing number of studies with animals and patients indicate that FGF21 is induced, in addition to starvation and obesity, by diverse pathogenic conditions such as liver injury, viral infection, chemical insult, specific nutritional deficiency, the hepatic regenerative response as well as liver diseases as hepatosteatosis, steatohepatitis, cirrhosis, and liver cancer (11–17). This evidence concerns the gene FGF21 and obesity due to melanocortin 4 receptor deficiency.