For gene therapists, the news in October that a child in a gene-therapy trial for severe combined immunodeficiency disease (SCID) had developed leukaemia was a devastating blow. The SCID trial, led by Alain Fischer at the Necker Hospital for Sick Children in Paris, was the field's most successful, having cured nine children of this life-threatening illness.

Fischer is now trying to figure out the cause of the tragedy, and whether it could happen again. It seems that his patient's cancer started when the retroviral vector that carried a corrective gene into the boy's body activated a gene called LMO-2, causing one of his cells to proliferate uncontrollably. The patient is now responding to treatment.

Gene therapists are quick to stress the differences between the current setback and the 1999 death of Jesse Gelsinger in a gene-therapy trial at the University of Pennsylvania in Philadelphia. Gelsinger was not expected to be cured by the treatment that killed him, and faults were later found in the management of the trial.

Although trials in several countries were put on hold after Fischer's announcement, a consensus seems to be emerging that they should resume — albeit with revised procedures for informed consent and improved monitoring of patients.

Germany, which halted all trials involving retroviral vectors in the light of earlier data from animal experiments, has already given the green light to some of its affected trials, and advisory committees to the US Food and Drug Administration and the National Institutes of Health have recommended restarting SCID trials in the United States.