Throughout adult life, all blood cells are constantly regenerated from a small pool of hematopoietic stem cells. A single purified stem cell injected into a lethally irradiated host is sufficient to reconstitute all lineages, demonstrating pluripotency and an enormous regenerative potential. Genes encoding secreted growth factors, signal transduction molecules and nuclear proteins regulate this process. The helical cytokine family comprises structurally related secreted proteins that play an important role as hematopoietic growth factors. Their cognate receptors are single transmembrane proteins that constitute the so-called cytokine receptor superfamily. One such ligand-receptor pair is Tpo (Thrombopoietin), also known as the c-Mpl ligand or MGDF (Megakaryocyte Growth and Development Factor) and its receptor, Tpo-R. Tpo is a hormone constitutively produced mainly by the liver and kidney and mediates biological [...]