Erythropoietin, otherwise known as EPO, is a hormone that increases aerobic power by stimulating the body’s production of red blood cells.

Undetectable as recently as 1999, synthetic EPO is the drug of choice among athletes in endurance sports. The first reliable test for it became available in 2000.

Several elite athletes have been suspended, banned — and in some cases cleared — after a failed test.

The most recent was Olympic sprinter Marion Jones, whose “A” sample tested positive at the U.S. track and field championships in June, according to people familiar with the results who spoke to The Associated Press on condition of anonymity Friday. Her “B” sample came back negative.

EPO is the substance seven-time Tour de France winner Lance Armstrong was accused of using by the Paris-based sports daily L’Equipe last summer. The paper reported that six of Armstrong’s samples — taken during his first win in 1999 and analyzed after the testing technology was developed — tested positive.

Armstrong angrily denied the report, and a Dutch investigator’s report later cleared Armstrong of the allegation; the paper stood by its story in an editorial. No disciplinary action was taken and Armstrong never tested positive for any performance-enhancers.

Other recent EPO cases:

• U.S. road cyclist Adam Bergman, who finished second in the Olympic trials in 2004, tested positive the following year for EPO and was suspended two years.
• Four-time Tour of Spain champion Roberto Heras was banned two years and stripped of his last title in February for a positive EPO test during stage 20 of the Spanish Vuelta — Spain’s most important cycling race — which he won in October.
• American sprinter Kelli White tested positive for modafinil at the 2003 World Championships in Paris and later admitted to using other performance-enhancers, including EPO.
• Alvin Harrison, a two-time Olympic gold medalist, accepted a four-year suspension in 2004 for admitting to using numerous undetectable performance-enhancing drugs, including steroids and EPO.
• Kenyan distance runner Bernard Lagat, a bronze medalist at the Sydney Games, was withdrawn from the world championships in 2003 by Kenyan officials after a positive drug test for EPO — but he was cleared in October when a test on his B sample was negative.

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