At 24, two entrepreneurs took on cancer. At 32, they’re worth hundreds of millions

In 2008, when he was 23, Nat Turner was on a hike in North Carolina with his 6-year-old cousin, Brennan Simkins. Brennan’s legs got weak, and the weakness kept getting worse. He turned out to have a rare and deadly pediatric leukemia that kept coming back after treatment. When Brennan needed a second bone marrow transplant, several hospitals refused to do it and his family was losing hope—until they found a specialist who would help.