Thrombopoietin, known also as TSF (thrombopoiesis stimulating factor), for mediator substances that function as natural stimulators of megakaryocytopoiesis and thus, promote the differentiation of platelets. Thrombopoietin stimulates an increase in the sizes and numbers of megakaryocytes. It also increases the number of small acetyl-cholinesterase positive cells that are early precursor cells of the megakaryocytic lineage. Thrombopoietin is a protein of 332 amino acids that shows significant homology with Epo (erythropoietin) in its N-terminal domain. A comparison of human and porcine Thrombopoietin sequences shows 83% identity between the erythropoietin-like domains but only 67% between the C-terminal domains.