In October 2017, Graham Hatfull received an urgent email from across the pond. A microbiologist colleague of his named James Soothill was desperately looking for a way to help two patients at the Great Ormond Street Hospital in London. The pair of teenagers, a girl and a boy, had cystic fibrosis, a genetic condition where the lungs can’t clear mucus or disease-causing bacteria. And they had both recently received double lung transplants as a result.The surgeries had gone well.