A seven-year-old US girl who was near death with leukemia now shows no sign of the disease after doctors treated her with HIV.
Emma Brooke-Whitehead had undergone extended chemotherapy without success when doctors at Children's Hospital of Philadelphia decided to try the groundbreaking new treatment about eight months ago.
"This treatment was really her only chance," pediatric oncologist Stephen Grupp told ABC News.
The treatment works by using a disabled version of HIV to retrain a patient's disease-fighting white blood cells to kill cancer cells.
Doctors removed millions of white blood cells from Emma, who was diagnosed with acute lymphoblastic leukemia at age five, and inserted the new gene before pumping them back into her body.
Dr Grupp said there was no risk of infection because "all of the things that make the HIV virus able to cause disease have been removed from this particular virus whose only purpose is to put a gene into a cell".
But the treatment is far from perfect, as the white blood cells also kill healthy cells.
After the procedure Emma became extremely ill but doctors gave her a rheumatoid arthritis drug that blocked the side effects and her condition soon stabilised.
Now eight months after the treatment doctors cannot find any traces of leukemia left in Emma.
"She has no leukemia in her body for any test that we can do — even the most sensitive tests," Dr Stephen Grupp said.
Emma has been in remission eight months but doctors say they need to wait about two years before they can deem her cured.
Mrs Whitehead said Emma looked and felt "amazing".
"She has a ton of energy. She's back with her class. She was even able to play a little bit of soccer," she said.
Only a dozen patients have had the treatment and only a few of those have been children.
Doctors say it has great promise as it has worked in cases that were deemed hopeless.
However, it has not cured all patients who have tried it, with two adults seeing no progress after treatment, one child relapsing and another four adults not going fully into remission.