"There have been deaths," Friedmann said.
And an otherwise successful attempt to cure severe combined immunodeficiency disorder—the so-called bubble-baby syndrome—was halted when some of the children developed leukemia.
Also the gene-therapy viruses that might lend themselves to cheating don't work as easily as had been hoped.
The problem is that the human immune system tries to fight them off, said H. Lee Sweeney, a physiology professor at the University of Pennsylvania.
"That's caused most of the trials to stop," Sweeney said.
In future tests patients may have to be hospitalized during treatment, with their immune systems suppressed.
"I'm not sure an athlete is going to be willing to be put in the hospital for six weeks right in the middle of their training," Sweeney said.
Less ambitious forms of gene doping may be right around the corner, though.
Dispensing with the troublesome virus-based delivery system, this type of doping would inject "naked" DNA directly into a muscle.
Nearby cells would take up some of the DNA, and if that DNA controls an important hormone, like EPO or human growth hormone (HGH), it might be enough to do the job.
It's not so different from injecting EPO or HGH directly, but it would save money, because it would only have to be done once.
"You could probably get a molecular-biology major to make it for you for a couple hundred dollars," Sweeney said.
Testing for this type of doping would be easy, though, since the athlete's body would still carry too much of the hormone.
Testing for full-blown gene doping will be more difficult. Just be safe, the International Olympic Committee is hanging on to Olympians' genetic samples for eight years, in case testing methods catch up with currently indetectable doping methods.
For its part, the World Anti-Doping Authority is working on a test to determine the expression of all 25,000 of the human body's genes, looking for abnormal patterns, said the University of California's Friedmann, who chairs the agency's genetics panel.
But sports authorities may eventually have to accept gene doping as a fact of life, scientists say.
The same techniques that could create superathletes will likely also help ordinary people stay fitter and healthier.
"I think [gene therapy] will change the way we all live and how health care treats the average person," the University of Pennsylvania's Sweeney said.
"You can't legislate it out of sport, because you'd be depriving people of a standard of care."
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