Compared to healthy controls (15.4 ± 2.0 pg/ml), apelin levels were reduced by about 50 % in patients with both earlier-stage (fibrosis) (6.5 ± 0.6 pg/ml) and later-stage liver disease (cirrhosis) (8.3 ± 1.2 pg/ml), indicating that changes in plasma apelin levels may be a useful biomarker for liver disease. The gene discussed is APLN; the disease is fibrosis.