Scientists say they are closer to understanding why a pioneering type of gene therapy led to two children developing leukaemia.
The treatment cured children such as Rhys Evans
The two cases in France, which were reported earlier this year, happened after the children had been cured of a rare and devastating immune disorder.
Experts writing in the journal Science say that the therapy activated a cancer-causing gene by mistake.
No similar UK cases have emerged, and both children responded to treatment.
The therapy was designed to tackle X-scid, often dubbed "baby in a bubble" syndrome.
This leaves affected children with no immune system, forcing them to live in a sterile environment, as minor infections could kill them. They often die at a very young age.
The gene therapy "patches" the defective gene which causes the condition, and, in this condition, has been remarkably successful in providing a cure.
However, the two cases of leukaemia, among just 11 children treated at the Necker Hospital in Paris, raised alarm among researchers.
They had suspected that cancer was one possible rare side-effect, but two such cases in close succession were not expected.
An investigation by scientists, including some from the Babraham Insitute and the MRC Laboratory of Molecular Biology in Cambridge, confirmed that the "implanted" gene patch also affected a nearby gene called LMO2.
This, when activated, proved to be a cancer-causing gene in these children, although it is believed that other unknown factors may have contributed to the disease.
Weighing the risks
The treatment is still allowed to go ahead in the UK, despite the cancer cases in France - and doctors at Great Ormond Street in London say it is still the best option for some patients.
A spokesman for the hospital said: "The only other option for children for whom there is no matched bone marrow donor is an unmatched donor - and we would expect four or five of the 16 patients treated so far to have died based on a 35% fatality rate at three years.
"We would still recommend that bone marrow transplantation is used where there is a matched donor."
The hospital advises any parent whose child faces this choice that there is an increased risk of cancer from this type of gene therapy, which must be weighed against the risks of other treatments - or no treatment at all.
Dr Terry Rabbitts, who helped investigate the cancer cases, said: "Although it is anguishing for parents to expose their children to the chance of developing cancer, the benefits of gene therapy for this devastating disease greatly outweigh the risks of the disease itself."
Professor Adrian Thrasher, who leads the Great Ormond Street programme, said that it might be possible to reduce the risk in future.
He said: "This goes some way to telling us the mechanism of the problem, which means that we can try to devise strategies to reduce this possibility.
"Some quite simple changes in design of the vector (the virus that carries the modified gene patch) may be all that is needed.
"Overall, the children treated in France and England continue to do really well."
One in 50,000 births in the UK is affected by X-scid, which only happens in boys.