A baby born with a rare and dangerous genetic disease is thriving after getting an experimental gene editing treatment made just for him. Researchers said the Pennsylvania boy is among the first to be successfully treated with a custom gene editing therapy. They described his case in a study published on Thursday in the New England Journal of Medicine. Though it may be a while before similar personalized treatments are available for others, doctors hope the technology can someday help millions who have been left behind by genetic medicine because their diseases are so rare.