Could inhibit both insulin-receptor tyrosine kinase activity and insulin-stimulated receptor autophosphorylation and, concomitantly, antagonize the mitogenic effect of the hormone in cultured rat hepatoma cells. Synthesized in liver and secreted by the hepatocytes in the blood. Undergoes complex post-translational modification involving N-glycosylation, and addition of fucose and sialic acid residues. Phosphorylation occurs at a serine residue. Phosphorylated by FAM20C in the extracellular medium. Belongs to the fetuin family.