US officials have approved the first treatment that genetically engineers patients' own blood cells to seek and destroy a childhood leukaemia.
The CAR-T cell treatment developed by Novartis and the University of Pennsylvania is the first type of gene therapy to hit the US market - and one in a powerful but expensive wave of custom-made "living drugs" being tested against blood cancers and some other tumours.
The Food and Drug Administration called the approval historic.
"This is a brand new way of treating cancer," said Dr Stephan Grupp of Children's Hospital of Philadelphia, who treated the first child with CAR-T cell therapy - a girl who had been close to death, but now is cancer-free for five years and counting.
"That's enormously exciting."
CAR-T treatment uses gene therapy techniques not to fix disease-causing genes, but to "turbo-charge" T cells, immune system "soldiers" that cancer can often evade.
Researchers filter the cells from a patient's blood, reprogramme them with a "chimeric antigen receptor" that targets cancer, and grow hundreds of millions of copies.
Returned to the patient, the cells can continue multiplying to fight disease for months or years.
Novartis did not immediately disclose the therapy's price, but it is expected to cost hundreds of thousands of dollars. It is made from scratch for every patient.
"We're entering a new frontier in medical innovation with the ability to reprogramme a patient's own cells to attack a deadly cancer," said FDA commissioner Scott Gottlieb.
This first use of CAR-T therapy is aimed at patients desperately ill with acute lymphoblastic leukaemia, which strikes more than 3,000 children and young adults in the US each year.
While most survive, about 15% relapse despite the current best treatments, and their prognosis is bleak.
In a key study of 63 advanced patients, 83% went into remission, although it is not clear how long the benefit lasts: Some patients did relapse months later.
The others still are being tracked to see how they fare long-term.