In the most striking success yet for gene therapy, French doctors have apparently cured four little boys of the rare and deadly immune system disorder dubbed “bubble boy” disease.

The boys, followed for up to 2 1/2 years, now have healthy immune systems, are growing normally and live at home after spending their first months in a sterile environment because they could not fight off disease.

The immune system of a fifth boy was partially restored, and he is improving with other treatment.

The doctors two years ago reported initial success with the first two patients; the update in today’s New England Journal of Medicine shows the treatment is still working.

“This is the first instance in which an otherwise fatal disease has been treated with gene therapy as the only treatment and the disease was reversed,” said Dr. Jennifer Puck, head of the immunologic genetics section at the National Human Genome Research Institute.

“This is exciting, and it’s important.”

The gene therapy was given to the boys in 1999 and 2000 when they were babies; the youngest was a month old, the oldest 11 months. The oldest of the boys is now about 4.

Puck, who was not involved in the research, said scientists are getting better at gene therapy experiments, but so far patients treated for other disorders still need regular, expensive medication to survive.

The study is welcome news for gene therapy field, which suffered a setback when an experiment killed a teen-ager at the University of Pennsylvania in 1999. The death raised fears that the experiments might be too dangerous and led to efforts to increase safety.

The five French boys were born with severe combined immunodeficiency, or SCID, an inherited disease that occurs in about 1 in 75,000 births. The best-known victim was David, Houston’s famous “bubble boy” who lived in a germ-proof plastic enclosure until his death at age 12 in 1984.