For the first time doctors have given kids' life-saving transplants of immune cells, engineered to carry suicide genes in case things go wrong.
3 kids at Great Ormond Street received the transplants that may have been considered too risky without what doctors called a genetic "insurance policy".
The kids needed bone marrow transplants and the doctors had not been able to find a perfect match and a normal transplant had risked life-threatening complications, with donor cells turning against the kids' body, Sky News reported.
So doctors tweaked cells from donors to carry a suicide gene and a unique 'flag' on their outer surface.
At the first sign of complication the doctors would have injected a drug that homed in on the flag, which would have triggered the suicide gene.
The cells would then have been destroyed before causing further damage.
Dr Waseem Qasim, a paediatric immunologist who led the study, said that they were reluctant to use certain donors as the risks of complications after a mismatch transplant are much higher
The technique has been published in the journal PLOS ONE.