Thrombopoietin is a lineage-restricted, glycoprotein growth factor ligand for the MPL receptor with selective action on megakaryopoiesis and platelet production. THPO has little direct effect on erythroid progenitor cells but may enhance erythropoiesis indirectly by preventing early erythroid progenitor cells from undergoing apoptotic cell death. (from OMIM 600044 and NCI)