Ethan Tepera was only 13 years old when he was diagnosed with chronic myeloid leukemia (CML) in October 2011. He’d been pale and feeling nauseated for several weeks. Then, the migraines began.

“Initially, the pain was manageable,” Ethan says. “But eventually, it got so bad I couldn’t sleep or tolerate any light whatsoever.”

Ethan’s mother took him to a pediatrician near their home in Dallas. That doctor discovered Ethan...