Imagine the parents' surprise when newborn screening revealed that one of their seemingly healthy newborn twins had a rare disease called severe combined immune deficiency (SCID) — popularly known as "bubble boy" disease.

Once his mother's lingering immune cells disappeared, little Levi Looks would have no immune system. An infection could kill him. After learning that Levi's fraternal twin was no match for a stem cell transplant, Kala and Phillip Looks enrolled their baby in a gene-therapy trial at Dana-Farber/Boston Children's Cancer and Blood Disorders Center led by hematologist Dr. Sung-Yun Pai.

Pai harvested Levi's bone marrow and sent his cells to the laboratory, where a so-called viral vector — a deactivated virus — inserted the corrected gene into his marrow cells' DNA. Four days later, Kala pushed a button with Phil right beside her, and the corrected cells were returned to their baby boy's bloodstream. "Just seeing that white marrow go in, and knowing it was changed was really something," Kala says.

Today both twins are healthy, lively toddlers. "Levi's blood work looks essentially normal for his age," says Pai. "You would never know the difference between him and his twin brother."