FGF21 and obesity due to melanocortin 4 receptor deficiency: Fibroblast growth factor 21 (FGF21) is expressed primarily by the liver and can be induced by stress and pathological process, such as steatosis.[33] In humans, serum concentration of FGF21 is increased in diabetes, obesity, NAFLD, and metabolic syndrome.[34] Hepatic Fgf21 expression and serum FGF21 levels (Figure 4G) were increased in WT mice following HFD feeding as expected, but not in the same treated bid−/− mice.