Erythropietin -EPO) is a glycoprotein hormone that is the primary regulator of erythropoiesis, maintaining the body s red blood cell mass at an optimum level. In response to a decrease in tissue oxygenation, EPO protein levels are upregulated in the kidney. The secreted hormone binds to specific receptors on the surface of red blood cell precursors in the bone marrow, leading to their survival, proliferation, differentiation, and ultimately to an increase in hematocrit.