University of Pennsylvania researcher H. Lee Sweeney has been testing a new gene treatment in mice. He injects the animals' muscles with viruses car- rying the gene for a substance called insulinlike growth factor-1 (IGF-1). This protein makes the muscles grow larger than average and heal more quickly after injury.

It even prevents the weakness that normally develops in the muscles of mice-and humans-as a result of age. An elderly mouse that had received the treatment in its youth had a muscle mass 60 percent greater than that of a normal mouse of the same age. "We showed that with a onetime injection of this gene we can get bigger muscles in young animals and that, as they get older, the muscles never change,"51 Sweeney told Sports Illustrated reporters in 2001.

As of mid-2001 Sweeney had no immediate plans to test his...