Thrombopoielin (TPO) is the key regulator of platelet production and is produced constilulively by the hepatocyte. It is secreted into the circulation where it is mostly bound and removed by thrombopoielin receptors (c-mpl) on platelets and megakaryocytes. The remaining thrombopoielin in the circulation regulates the growth of bone marrow megakaryocytes and the production of platelets. With a decline in platelet production the circulating concentration of thrombopoietin rises. The human thrombopoielin glycoprotein consists of an erythropoietin-like domain (residues 1-153) that is 50% similar to erythropoietin and a carbohydrate-rich domain (residues 154-332) that is highly glycosylated and appears to be necessary for secretion and stability of the mature protein.