Under the condition of obesity and insulin resistance, excessive influx of FFAs to the liver would induce FGF21 over-expression, and then elevated FGF21 could in turn decrease the level of serum FFAs through the inhibition of lipolysis in WAT [29]and inhibit the hepatic triglycerides generation and hepatic steatosis through promotion of fatty acid oxidation and ketogenesis [30]. The gene discussed is FGF21; the disease is obesity due to melanocortin 4 receptor deficiency.