Getting a speed boost from DNA. (Turbo Gene).

Whether you're better suited to run a marathon or a 100-meter sprint correlates with a gene called ACTN3, researchers find.

The gene encodes the protein [alpha]-actinin-3, which functions in the so-called fast-twitch muscles. These muscles give the extra power needed for brief and vigorous bursts of activity, such as sprinting and speed skating.

Previous studies revealed that about one out of five white people in Australia has a variant of the ACTN3 gene that cannot yield [alpha]-actinin-3. Because people with the deficiency seem healthy, the protein appears to be unimportant for day-to-day activities. Most likely, it's significant only "in the extremes of performance," says Kathryn North of the Children's Hospital at West-mead in Sydney, Australia.

That's where the athletic connection comes in. North and her colleagues theorized that because the protein affects fast-muscle fibers, a deficiency of [alpha]-actinin-3 might be detrimental to speedy running and thus less likely to occur in elite sprinters.

To test their theory, the researchers genetically screened a group of world-class athletes, including Australian Olympic competitors, and a control group of nonathletes. All the participants were white. The scientists divided the athletes into two groups: sprinters, which included speed skaters, and endurance athletes, such as marathon runners and rowers.

Of the sprinters, 6 percent had the gene variant leading to [alpha]-actinin-3 deficiency. That's one-third the rate for nonathletes, 18 percent of whom carried the variant. With a rate of 24 percent, endurance runners were similar to the nonathletes. The findings will appear in the September American Journal of Human Genetics.

"The unique finding of this study is that the sprint athletes are different," comments Tuomo Rankinen of the Pennington Biomedical Research Center at Louisiana State University in Baton Rouge. "The endurance athletes do not really differ from the nonathletes in terms of this genetic marker."

There is, however, a gender difference. None of the female sprinters was [alpha]-actinin-3 deficient. North thinks that in male sprinters lacking the protein, hormones such as testosterone may compensate for the protein deficiency.

Previous studies have shown ethnic differences. Some 25 percent of Asians are [alpha]-actinin-3 deficient, compared with only 1 percent of Bantu-speaking people in Africa. North says that the importance of [alpha]-actinin-3 protein for athletic performance may vary in different human populations.

"We know that at the moment, the best sprinters in the world are black," Rankinen says. "The next question is to compare the frequency of these ACTN3 genotypes in black sprinters with [that of] the general African-American population, for example."

But North's first task is to establish how the gene's protein product, [alpha]-actinin-3, works in fast-twitch muscles. After knocking out mouse ACTN3 genes, "we're going to be putting mice through little sprinting tests" and looking for structural changes in the muscles, she says.