Lineage-specific cytokine affecting the proliferation and maturation of megakaryocytes from their committed progenitor cells. It acts at a late stage of megakaryocyte development. It may be the major physiological regulator of circulating platelets. Two-domain structure with an erythropoietin-like N-terminal and a Ser/Pro/Thr-rich C-terminal. Belongs to the EPO/TPO family.