PORTLAND, Ore. — By the time her mother received the doctor’s email, Yuna Lee was already 2 years old, a child with a frightening medical mystery. Plagued with body-rattling seizures and inconsolable crying, she could not speak, walk or stand.

“Why is she suffering so much?” her mother, Soo-Kyung Lee, anguished. Brain scans, genetic tests and neurological exams yielded no answers. But when an email popped up suggesting that Yuna might have a mutation on a gene called FOXG1, Soo-Kyung froze.

“I knew,” she said, “what that gene was.”

Almost no one else in the world would have had any idea. But Soo-Kyung is a specialist in the genetics of the brain—“a star,” said Robert Riddle, a program director in neurogenetics at the National Institute of Neurological Disorders and Stroke. For years, Soo-Kyung, a developmental biologist at Oregon Health and Science University, had worked with the FOX family of genes.

“I knew how critical FOXG1 is for brain development,” she said.

She also knew harmful FOXG1 mutations are exceedingly rare and usually not inherited — the gene mutates spontaneously during pregnancy. Only about 300 people worldwide are known to have FOXG1 syndrome, a condition designated a separate disorder relatively recently. The odds her own daughter would have it were infinitesimal.