Thrombopoietin (TPO), also known as thrombopoiesis stimulating factor (TSF), is a glycoprotein hormone and the major stimulator of megakaryopoiesis and platelet production. TPO is expressed in liver, kidney, spleen, lung, bone marrow, and brain. The TPO receptor is a product of the proto-oncogene
c-mpl and displays homology with type I cytokine receptor superfamily members. Analogous to the effect of erythropoietin (EPO), the primary mode of action of TPO is inhibition of apoptosis of its target cells. By contrast, TPO is strongly proapoptotic in the brain and acts as a counterpart of EPO which has neuroprotective properties.