GDF15 and hepatocellular carcinoma: Using the optimized parameters determined above in the established RPPA system, we quantitatively measured the expression levels of 10 proteins including AFP, B2M, CA15-3, CEA, GDF-15, GP73, HE4, IGFBP-2, OPN and PDGF-Rb in serum samples of 132 hepatocellular carcinoma (HCC) patients and 78 healthy controls.