The doctors looking at Layla Richards saw a little girl with leukemia bubbling in her veins. She’d had bags and bags of chemotherapy and a bone marrow transplant. But the cancer still thrived. By last June, the 12-month-old was desperately ill. Her parents begged—wasn’t there anything?
There was. In a freezer at her hospital—Great Ormond Street, in London—sat a vial of white blood cells. The cells had been genetically altered to hunt and destroy leukemia, but the hospital hadn’t yet sought permission to test them. They were the most extensively engineered cells ever proposed as a therapy, with a total of four genetic changes, two of them introduced by the new technique of genome editing.