The chances of it hitting any one gene are small, because the virus is only about 5,000 base pairs long – about 1/100th the size of the genome.There is a chance that it could insert itself into a working gene, and deactivate it or create problems, said Bobby Gaspar, a consultant immunologist on the Great Ormond Street team "Leukaemia is a finite risk of gene therapy trials. But this and a trial in Science with some mice which was a special case are the only known occurrences ... The chances are somewhere in the range of 1 in 10 million to 100 million."This appears to have been one of those chances. The virus inserted itself near a gene known as "LmO2", which is important for generating blood cells.

Leukaemia is cancer of the blood, so it seems that the virus may have turned on the LmO2 gene wrongly. The child is responding to chemotherapy.This might look like another blow to gene therapy, which was hyped as the perfect means of curing almost any illness.The first treatment to use it came in September 1990. It was used in the US on Ashanthi DeSilva, four, who had a condition called ADA, which also affects the immune system It appeared to work. Optimism about gene therapy rocketed but her long-term survival probably had more to do with the drugs she kept taking.Hopes for gene therapy at that time were sky high, but the fervour dimmed when trials produced results that were at best equivocal and at worst negative. Britain's first gene-therapy operation came in 1993 – the year the advisory council was set up – on Carly Todd, when she was 17 months old.

That proved disappointing, although Carly is doing well after conventional treatment and a bone-marrow transplant.The low point was in September 1999, when 18-year-old Jesse Gelsinger from Arizona died four days after starting trials of a gene therapy treatment for an inherited metabolic disorder. The suspicion was that the virus used to carry the genes had reacted with other cells in his body; he showed the signs of a severe immune reaction.In the ensuing investigations, American researchers were found to be under-reporting the "adverse events" (up to and including deaths) of gene therapy. In the US, funding dried up, and applications to use gene therapy slumped.In the UK, the Gene Therapy Advisory Council made its own review of the evidence, deciding to allow researchers to persist in their pursuit of this holy grail. Even as the bad news was emerging about Mr Gelsinger, the trials were starting in France to treat X-SCID.And now gene therapy is starting to come out of its long clinical exile, as a potential, and lasting treatment for illnesses where conventional therapies are limited or rely on donations that may never arrive.

To date, there have been 636 completed, ongoing or pending gene therapy clinical trials worldwide. The method varies: sometimes the genes are injected into the body directly (as with Jesse andAshanthi), or sometimes, as with Rhys and the French children, by extracting cells, treating them and reinserting them.The successes are starting to make their mark. "Five years ago, there were no successful gene therapies," said Dr Gaspar "Now, we have cures. X-SCID was the first, but there have also been successes with ADA, where two children have been cured in Italy. These are the first steps to gene therapy for a wide range of diseases."CONDITIONS IN THE QUEUE FOR THERAPYX-linked SCID Severe combined immunodeficiency (SCID) linked to defective genes on the X-chromosome is an inherited disorder affecting boys.