Erythropoietin, or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production. It is a cytokine for erythrocyte (red blood cell) precursors in the bone marrow. Also called hematopoietin or hemopoietin, it is produced by the peritubular capillary endothelial cells in the kidney, and is the hormone that regulates red blood cell production. The existence of a hormone that controls RBC production was first suggested by the experiments of Paul Carnot in 1906, who created anemic rabbits and then transfused their serum into recipient rabbits. EPO is produced by peritubular cells in the adult kidney, and in hepatocytes in the fetus. In adults, a small amount is also produced by the liver. The rate of Epo synthesis and secretion depends on local oxygen concentrations; hypoxia is the main stimulus for Epo production. Although the use of erythropoietin has been studied in critically ill patients, erythropoietin has not been shown to be effective in this setting. In a randomized controlled trial, erythropoietin insignificantly reduced mortality among critically ill patients. In 1983, the gene coding for EPO was identified, leading to its synthesis as epoetin-alfa by American genetic research corporation, Amgen, who patented the drug under the name Epogen. In 1989, another company, Ortho Biotech, a subsidiary of Johnson and Johnson, began marketing the drug under license as Procrit in the US, and Eprex in the rest of the world.