The results that FGF21 is primarily expressed in the liver in response to fasting and pathological conditions such as fatty accumulation and the obesity and diabetes [68], [69], and that FGF21 targets primarily the fatty tissues, indicate an emerging endocrine metabolic pathway from the liver to fatty tissues in regulating the lipid, glucose and energy metabolic homeostasis. The gene discussed is FGF21; the disease is obesity due to melanocortin 4 receptor deficiency.