Treatment of the anemia of renal failure has been revolutionized by the use of erythropoietin and other ESAs (erythropoiesis-stimulating agent). Concerns with ESA use include a substantial number of End Stage Renal Disease (ESRD) patients with ESA-resistant anemia, and a growing body of evidence of potential negative effects of high doses of ESA use, including increased mortality and increased rate of tumor growth in cancer patients.