At high altitudes, there is a decrease in oxygen hemoglobin saturation. This hypoxic condition causes hypoxia-inducible factor 1 (HIF1) to become stable and stimulates the production of erythropoietin (EPO), a hormone secreted by the kidneys, EPO stimulates red blood cell production from bone marrow in order to increase hemoglobin saturation and oxygen delivery. Some athletes demonstrate a strong red blood cell response to altitude while others see little or no gain in red cell mass with chronic exposure. It is uncertain how long this adaptation takes because various studies have found different conclusions based on the amount of time spent at high altitudes.While EPO occurs naturally in the body, it is also made synthetically to help treat patients with kidney failure and to treat patients during chemotherapy. Over the past thirty years, EPO has become frequently abused by competitive athletes through blood doping and injections in order to gain advantages in endurance events. Abuse of EPO, however, increases RBC counts beyond normal levels (polycythemia) and increases the viscosity of blood, possibly leading to hypertension and increasing the likelihood of a blood clot, heart attack or stroke. The natural secretion of EPO by the human kidneys can be increased by altitude training, but the body has limits on the amount of natural EPO that it will secrete, thus avoiding the harmful side effects of the illegal doping procedures.