For 85% of kids with a terrible cancer called acute lymphoblastic leukemia, chemotherapy is a cure–but not for Emily Whitehead. Diagnosed at 5, she suffered an infection from her first round of chemo and nearly lost her legs. Then the cancer came back; she was put into remission once more and scheduled for a bone marrow transplant. As she waited, the cancer returned yet again. There was nothing else to try.

Nothing except a crazy experimental treatment never before given to a child: Blood was taken out of 6-year-old Emily’s body, passed through a machine to remove her white cells and put back in. Then scientists at the University of Pennsylvania used a modified HIV virus to genetically reprogram those white cells so that they would attack her cancer, and reinjected them.