A seven-year-old girl has become the first child leukaemia patient to be successfully treated by doctors using a disabled form of the virus that causes Aids to reprogram the immune system.
When chemotherapy failed to work for Emily Whitehead, diagnosed with acute lymphoblastic leukemia, she underwent a new experimental treatment at The Children's Hospital of Philadelphia.
It involved tricking her immune system into fighting cancer the cancer cells - and, six months later, Emily remains in remission.
She was one of a dozen people to have had the treatment. Three adults also had complete remissions, with two of them now clear of cancer for more than two years.
Four other adults showed improvements but did not go into complete remission. One child improved but but then relapsed, and the treatment did not work for two adults.
The patients each had millions of T-cells removed. Using the disabled form of HIV, these were modified to attack cancer cells before being put back into their systems.
The patients became very ill with feverish symptoms before recovering, and Emily almost died. She described the experience as "really, really scary" - but has recovered enough to go back to school.
Researchers said the treatment, which costs about $20,000 for each patient, was in the early stages but they hope it could ultimately replace risky bone marrow transplants.
Pediatric oncologist Dr Stephan Grupp, Director of the Center for Childhood Cancer Research at The Children's Hospital of Philadelphia, told CBS: "We've treated the first couple of patients and we've been blown away by the results. They've been very exciting.
"We collect cells of the immune system from a patient, so we use the patient's own cells. We put in a new gene in those cells that makes the cells go after cancer cells and then we put those cells back in the patient.
He added: "We don't know until we treat more patients and we have longer follow up, whether there's a potential for curing these patients.
"Previous patients got sick, but it wasn't clear at the time whether it was due to the T-cells or an infection.
"Now we know the main reason they were sick was the cells. But now we can intervene. She (Emily) taught us."
Her father Tom Whitehead, from Philipsburg, Pennsylvania, told the Philadelphia Inquirer: "In the war against cancer there is no one in front of Emily."