“Marathon mice,” genetically engineered by Howard Hughes Medical Institute researchers, can run twice as far as their unaltered buddies. Previously, the only known way to increase endurance was through training. With no previous running experience, most mice can run about 900 meters before exhaustion. But the genetically altered mice can run 1800 meters (more than a mile_ before running out of steam, and keep it up for two and a half hours — an hour longer than unaltered mice can run.