In the 1910s, a relatively unknown cancer researcher named Maud Slye announced the first results of a study with the loftiest ambitions: to identify what causes cancer. To answer that question, the University of Chicago geneticist had bred tens of thousands of mice, enough to fill a three-story building. She carefully documented their ancestry and their morbidities and performed autopsies. And to Slye, her findings were clear: vulnerability to cancer was hereditary. If we wanted to, we could eliminate it. But Slye made some crucial mistakes along the way—and a number of enemies.