Thrombopoietin (TPO) is produced primarily in the liver and secondarily in the kidneys and bone marrow. It can bind to a receptor (which is the proto-oncogene c-Mpl, a homolog of an envelope protein of the myeloproliferative leukemia virus) on primitive blood cells and megakaryoctes and stimulate those cells to grow (proliferate) and evolve into platelet-producing megakaryocytes.