The Summer Olympics have finally begun! Time to celebrate the extraordinary talent, fortitude and grace of athletes representing the world's diverse nations, from Iceland to Chile. And time to wonder how many competitors are ingesting performance-enhancing substances.

When I started writing about science in the early 1980s, we worried whether athletes were amped up on amphetamines or steroids. Kid stuff. Now Olympics officials fear that some contestants may be boosting their strength, speed and endurance by fiddling with the core of their biological being, the genome.

Since the late 1990s, researchers have shown that, by inserting genes into mice and other animals, they can swell the animals' muscle mass, help cells repair themselves faster and boost the production of oxygen-toting red blood cells. The genes can be injected directly into the muscles of the target animal or slipped into the animal's own DNA by means of viruses.

One pioneer of this research is University of Pennsylvania geneticist H. Lee Sweeney, who described his work in Scientific American in 2004. In the late 1990s, he inserted the so-called IGF-1 (for insulin growth factor) into mice, boosting their strength by as much as 27 percent. Sweeney and other researchers hope that these sorts of gene therapies can benefit the elderly, patients suffering from muscular dystrophy and even astronauts who lose muscle mass in a zero-gravity environment.

Naturally, athletes—after reading media reports about "Schwarzenegger mice"--became interested in gene therapies too. Ever since his work was first publicized, jocks have been begging Sweeney to test gene therapy on them. He always turns them away, but he predicts that athletes may eventually find someone who will administer gene therapy to them, just as they have found suppliers for steroids and other illegal performance-enhancers. In principle, gene therapy can be aimed at specific targets, such as fast-twitch or slow-twitch muscle cells, to benefit specific athletes, say, a shot-putter, weight-lifter, sprinter or high-jumper.

The World Anti-Doping Agency, which advises the Olympics on cheating, fears that gene therapy would be extremely difficult to detect with conventional blood tests, because neither the novel gene, its viral carrier nor the proteins generated by the gene show up in blood or urine tests. The only way to test for gene doping would be to carry out muscle biopsies, a step that, needless to say, many athletes would resist, because it calls for removing a chunk, albeit tiny, of muscle.

So how concerned should we be that medalists of this summer's Olympics may be genetic cheats? Not very, because gene therapy for humans always fallen far short of its hype. Just two decades ago, proponents were predicting that gene therapy would soon eliminate many diseases, from cystic fibrosis to early-onset breast cancer, traceable to a defective gene. Some enthusiasts even foresaw the advent of genetically engineered "designer babies" who would grow up to be smarter than Nobel laureates and more athletic than Olympians.

Needless to say, these scenarios never quite materialized. Yes, China approved a gene therapy for cancer in 2004, and Europe may soon permit commercialization of a therapy for a disease that renders victims incapable of metabolizing fat, according to a recent report in Nature. But in the U.S., where researchers have carried out more than 1,000 clinical trials of gene therapy, not a single one has been approved for sale. In fact, U.S. researchers had to scale back their ambitions after several widely publicized deaths of patients enrolled in trials, notably Jesse Gelsinger in 1999.

Gene therapy can provoke a dangerous response from the body's immune system, and viruses used to introduce genes into the body can cause cancer. For these reasons, the National Institutes of Health warns that "the technique remains risky and is still under study to make sure that it will be safe and effective."

There may be a few wannabe Olympians out there who risk their careers, health and lives by experimenting with unproven gene doping. But I doubt that genetic engineering will have a significant impact on the Olympics anytime soon. When it does, that should, perhaps, be cause for celebration, because it would mean that gene therapy is finally beginning to fulfill its enormous potential.

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Addendum: This post is adapted from a column originally published in BBC Knowledge.