"This clearly is not a coincidence," Noguchi told reporters at a meeting of a panel of experts advising the FDA on the issue.

The FDA is trying to decide whether to stop similar trials on children with immune deficiencies, or whether their risk of dying from immune defects outweighs any risk of leukemia.

Noguchi said there was no evidence anyone else had developed cancer from gene therapy. He said 150 different gene therapy experiments in the United States had used retroviruses--the same kind of virus used in the French trial and the virus blamed for causing the boy's cancer.

"We are actively reviewing not just (new) reports that come in, but we are re-reviewing reports for all other trials," he said. "So far we have not seen anything yet of a leukemia or a lymphoma in a person that was not present before being treated."

But experts at the meeting said it was possible other gene therapy experiments could cause cancers because many different genes can cause cancer when disrupted during the therapy.

The FDA said last week it had halted three gene therapy trials upon learning that the French boy had developed leukemia after being treated for X-linked severe combined immunodeficiency or X-linked SCID, also known as "bubble boy" disease.

SCID patients are born without any working immune system. Without treatment, they once lived out their short lives in sterile "bubbles" because any infection would kill them. A single genetic defect causes the syndrome.

The condition can be treated with bone marrow transplants, which are highly successful if done very early and with bone marrow from a relative whose tissue matches closely. But babies with no healthy matched relatives have fewer options.

The gene therapy, which reinforces the children's bone marrow with genetically engineered immune cells, had worked well in the children. Ten boys in France treated using gene therapy were living normal lives.

The French researchers, led by Dr. Alain Fischer of Necker Hospital in Paris, used a retrovirus to carry normal copies of a gene to correct the defect in bone marrow stem cells, and then infused the cells back into the boys.

Retroviruses insert their own genetic material into the DNA of the cells they infect. The corrected bone marrow cells grow and proliferate to replace defective cells.

For gene therapy, the virus is crippled so it cannot reproduce. But researchers cannot yet control where the virus puts its genetic material.

During gene therapy, tens and even hundreds of millions of cells are infected with the genetically engineered virus. The retrovirus integrates into random parts of the genome--the entire collection of genetic material.

It seems that when Fischer's team infected the boy's bone marrow cells with the virus, one virus integrated itself into a sensitive point on chromosome 11 in a single cell.

This was near a gene called LMO-2, which controls the replication of cells.

LMO-2 causes embryos to grow many of their blood cells, Dr. Christof Kalle of the University of Cincinnati, who analyzed the French child's blood cells, told the meeting. The virus probably reactivated this process, which caused leukemia.

Fischer said the boy was doing well on chemotherapy. He said the family had a history of cancer--the boy's sister had a brain tumor. And the boy had recovered from two viral infections, including chickenpox, which may have made him more susceptible to leukemia.

The other boys in the trial have been checked and do not have a similar overgrowth of blood cells.