Their genes also have caused the two men enormous suffering. Pete’s parents first realized something was wrong when, as a teething baby, their son almost chewed off his tongue. “That was a giant red flag,” says Pete, now 34 and living in Kelso, Wash. It took doctors months to figure out he had congenital insensitivity to pain, caused by two different mutations, one inherited from each parent. On their own, the single mutations were benign; combined, they were harmful.

Dreyer, who lives in Johannesburg, was 21 months old when his parents noticed a sudden facial paralysis. Doctors first diagnosed him with palsy. Then X-rays revealed excessive bone formation in his skull, which led to a diagnosis of sclerosteosis. Nobody in Dreyer’s family had the disorder; his parents both carried a single mutation, which Dreyer inherited.

Dreyer and Pete are “a gift from nature,” says Andreas Grauer, global development lead for the osteoporosis drug Amgen is creating. “It is our obligation to turn it into something useful.”