When Cory Johannessen was a graduate student at Harvard Medical School, he started getting very exciting results. The anti-cancer drug he and his colleagues were testing in mice was working well, and was on the path toward clinical trials in people. But Johannessen’s excitement gave way to frustration. As he finished up graduate school, the cancer came back in the mice. The clinical trial hadn’t even started, and already Johannessen could see what might happen in patients: some would get better at first, but eventually their cancer would find a way to mutate and become resistant.