At the 1964 Winter Olympics, Eero Mäntyranta won the 15 kilometer cross-country skiing competition by a whopping 40 seconds—a margin of victory that has never been equaled. That same year, he won the 30 kilometer race by a full minute. So what made this legendary Finnish skier such a success? According to sports journalist David Epstein, Mäntyranta became the "greatest endurance athlete" of his generation in part because of a single mutation to his erythropoietin receptor (EPOR) gene, which helps regulate the production of red blood cells.