Scientists in the US and France have suspended trials of the first gene therapy to ever work because of a side effect.
The treatment had appeared to cure a French boy with a rare immune disorder dubbed "bubble boy disease".
It is unclear if the gene therapy actually caused the boy's new illness which is said to be similar to leukaemia.
The boy, a 2½-year-old, received gene therapy in his first month of life that worked very well - he fought off an infection last spring that otherwise could have been life-threatening.
But in late summer his body suddenly overproduced white blood cells, a reaction doctors called leukaemia-like. He is responding well to chemotherapy, scientists said.
Nine other children given the same gene therapy at Paris' Necker Hospital are doing well, and no other children worldwide given gene therapy for this disease - severe combined immunodeficiency, or SCID - have ever shown such a side effect.
But France's public health agency announced today that all the families had been notified about the possible risk.
US regulatory officials have put on hold three SCID clinical trials, one that had treated four children and two others that had been poised to start soon.
Several US patients enrolled in similar gene therapy studies dating back 10 years have also been contacted to have the possible risk explained, said Dr Phil Noguchi of the US Food and Drug Administration.
The FDA's scientific advisers will evaluate the development, in consultation with the scientists from Paris, at a meeting next week to determine what additional research or precautions are needed before the gene therapy studies can resume.
PA