Scientists are showing off "marathon mice" that have been genetically engineered to run faster and farther, with implications for humans involving strength and health. Lead researcher Ronald Evans of San Diego's Salk Institute calls it "a real breakthrough in our understanding of exercise and diet and their effects on obesity." Evans says genetic modification of what he calls the body's "fat switch" could have an impact on human disorders from obesity to heart disease. The gene engineered in the mice essentially mimics exercise, conferring endurance and preventing obesity even when the rodents were kept inactive and fed a high-fat diet. The genetically enhanced marathon mice ran twice as far and nearly twice as long as naturally bred rodents. Evans says he's seen no bad side effects.