A 10-year study of nine boys born without the ability to ward off germs has found that gene therapy is an effective long-term treatment, but it carries a price: four of them developed leukemia.

The technique is designed to help boys with X-linked severe combined immunodeficiency disease, or SCID, a rare mutation that prevents the body from making mature T cells or natural killer cells, which are vital tools for fighting infections.

Without a bone marrow transplant, which works best with a matching donor, such "bubble babies" have to live in germ-free environments and usually die within a year. Doctors hope gene therapy will work when no donor is available.

But its success has been limited by the threat of leukemia, and the new study looks at the long-term impact of a treatment that the researchers last used in 2002.

The team, led by Salima Hacein-Bey-Abina of the Paris Descartes University, reported in the New England Journal of Medicine that the treatment provided a long-term cure for SCID, in most cases.

"All children except one, including the three survivors of T-cell acute leukemia, could live normally in a nonprotected environment and cope with microorganisms without harmful consequences while growing normally," they concluded.

The median age of the boys when they got the gene therapy was seven months. The one death was from leukemia.

Leukemia is much easier to treat in young children than SCID. The success rate is usually 90 percent or more.

The team said the research has set the stage for tests of other types of gene therapy that use safer methods for correcting defective DNA.