As we have shown that the FGF21 concentration in patients with mild or moderate hepatic steatosis was elevated in parallel with serum ALT level, but this balance would break in severe NAFLD patients, whose biochemical indexes will show an obviously elevated ALT concentration but only a slight unparallel elevation of FGF21 concentration probably due to the presence of hepatic injury. This evidence concerns the gene FGF21 and fatty liver disease.