- Soluble thrombopoietin receptor (Mpl) and granulocyte colony-stimulating factor receptor directly stimulate proliferation of primitive hematopoietic progenitors of mice in synergy with steel factor or the ligand for Flt3/Flk2. Ku, H., Hirayama, F., Kato, T., Miyazaki, H., Aritomi, M., Ota, Y., D'Andrea, A.D., Lyman, S.D., Ogawa, M. Blood (1996)