EPO is predominantly synthesized and secreted by tubular and juxtatubular capillary, endothelial, and interstitial cells of the kidney. Approximately 10-15% of the total amount of EPO comes from extrarenal sources and is predominantly produced by hepatocytes and Kupffer cells of the liver. Approximately 40% of the molecular mass of EPO is due to its glycosylation. Glycosylation is an important factor determining the pharmacokinetic behaviour of EPO in vivo. Non-glycosylated Epo has an extremely short biological half life. Recombinant Human EPO is a glycosylated protein that runs at approximately 35 kDa owing to its glycosylation.