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The Food and Drug Administration yesterday suspended 27 gene therapy trials involving several hundred patients after learning that a second child treated in France had developed a condition resembling leukemia.

The agency said it was not aware that any of the patients treated in the 27 American trials had suffered illnesses similar to that of the infants in France but was nevertheless taking precautions.

"We see no evidence that the subjects in these 27 trials are actually at risk," said Dr. Philip Noguchi, acting director of the agency's office of cellular, tissue and gene therapies.

The temporary halt, the largest such action involving gene therapy trials, is yet another setback to the fledgling field, which usually involves introducing healthy genes into patients to treat diseases caused by defective ones. The field is still shaken from the death of a teenager undergoing gene therapy in 1999 at the University of Pennsylvania and from the first case of leukemia in an infant in France last year.

The treatments in France had been considered the only unequivocal success for gene therapy after a...