Erythropoietin - EPO.

Erythropoietin (EPO) is a hormone produced by the kidney that promotes the formation of red blood cells by the bone marrow. The resultant rise in red cells increases the oxygen-carrying capacity of the blood. 

The kidney cells that make EPO are specialized so that they are sensitive to low oxygen levels in the blood coming into the kidney. These cells make and release EPO when the oxygen level is too low.

EPO is a protein, with an attached sugar (a glycoprotein). It is one of a number of similar glycoproteins that serve as stimulants for the growth of specific types of blood cells. 

Normal levels of EPO are 0 to 19 (some say up to 24) mU/ml (milliunits per milliliter). Higher levels might indicate that an athlete has been abusing EPO for a competitive advantage.

 EPO has been misused as a performance- enhancing drug in aerobic sports such as cycling (in the Tour de France), long-distance running, speed skating, and Nordic (cross-country) skiing. When misused in such situations, EPO is thought to be especially dangerous (perhaps because dehydration due to vigorous exercise can further increase the thickness (viscosity) of the blood, raising the risk for heart attacks and strokes. EPO has been banned by the Tour, the Olympics, and other sports organizations.

For the latest stories about EPO abuse, see here.