Thrombopoietin is a glycoprotein class 1 hematopoietic cytokine, produced primarily in the liver. It is a critical regulator of hematopoiesis. It acts through the thrombopoietin receptor which is expressed on hematopoietic stem and progenitor cells. Action results in a number of signal transduction events that prevent apoptosis, improve cell viability, promote growth, and possibly increase differentiation.