Thrombopoietin (TPO) is a key regulator of megakaryocytopoiesis and thrombopoiesis in vitro and in vivo. TPO stimulates the proliferation and maturation of megakaryocytes and has an important role in regulating the level of circulating platelets in vivo (Bartley et al.; de Sauvage et al.; Foster et al.; Sohma et al.). TPO also promotes the survival, self-renewal, and expansion of hematopoietic stem cells and primitive multilineage progenitor cells. It is commonly used with other cytokines such as stem cell factor (SCF) and Flt3/Flk-2 ligand to promote expansion of primitive hematopoietic cells in culture (Hitchcock and Kaushansky). The TPO receptor, c-Mpl, is expressed at all stages of megakaryopoiesis, from hematopoietic stem and progenitor cells to mature platelets (Ng et al.).