Glycopeptide hormone which is the major physiological regulator of platelet production. Promotes the maturation of megakaryocytes and increases platelet size and number. Produced in the kidney; detectable in plasma and urine of thrombocytopenic animals and humans. Identification of humoral factor that increases platelet levels: E. Kelemen et al., Acta Haematol. 20, 350 (1958). Partial purification from rabbit plasma: B. L. Evatt et al., J. Lab. Clin. Med. 83, 364 (1974); from human plasma: A. Vannucchi et al., Leukemia 2, 236 (1988). Production by cultured human embryonic kidney cells: T. P. McDonald et al., J. Lab. Clin. Med. 85, 59 (1975). Purification: idem et al., Exp. Hematol. 17, 865 (1989). Review of biological activity and properties: idem, Am. J. Pediatr. Hematol. Oncol. 14, 8-21 (1992). Recombinant TPO acts both as a proliferative and a maturation factor for megakaryocytes. Cloning and characterization of human TPO: F. J. de Sauvage et al., Nature 369, 533 (1994); of murine: S. Lok et al., ibid. 565; K. Kaushansky et al., ibid. 568; F. Wendling et al., ibid. 571.