Fibroblast growth factor 21 (FGF-21) is expressed in many tissues, including the liver, brown adipose tissue, white adipose tissue, and pancreas. In mice, circulating FGF-21 originates from the liver. FGF-21 promotes glucose absorption by adipocytes, but has no effect on other types of cells. The serum FGF-21 levels in patients with type II diabetes mellitus (T2DM) increased significantly, indicating its role in the pathogenesis of T2DM. Elevated FGF-21 is also associated with liver fat content in non-alcoholic fatty liver disease, and is positively correlated with human BMI, indicating that obesity is a result of tolerance to FGF-21.