Doctors genetically engineered "Schwarzenegger Mice" and "Marathon Mice" in separate experiments conducted seven years apart. They made the former to research muscular dystrophy treatments and the latter to study obesity and heart disease, but now applications for athletes are expected.

Schwarzenegger Mice were unveiled in 1997 by researchers at Johns Hopkins. They had their myostatin genes — the gene that regulates muscle growth — removed and grew twice as muscular as siblings with the gene intact.

Marathon Mice were unveiled in 2004 by researchers at the Salk Institute in San Diego. Lead researcher Ronald Evans discovered what he called the "fat switch" gene more than 10 years earlier. His team found that mice ran farther and longer if that gene was switched on continuously. So they did that. Even the laziest mice dropped weight and increased endurance.

The question now is whether experiments on athletes can mimic experiments on mice. But since clinical trials of gene therapy for enhancement purposes are illegal, there is no way for researchers to conduct them safely.