Alyssa Tapley was 13 when doctors told her she was going to die. She was diagnosed with a rare form of leukemia and had endured a year of failed treatments, including chemotherapy and a bone marrow transplant, when her doctors said they had run out of options. They were talking about end-of-life care when Tapley begged them not to give up on her. One of her doctors read about research into gene editing being done by Harvard professor David Liu, and thought it may help the teen. Tapley, now 16, was the first human to try the experimental treatment, made...