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ene therapy is a catch-all term for efforts to treat an illness by altering the genes inside a patient's cells.

A new gene may be delivered into cells to repair an inherited genetic defect, treat diseased cells directly or, as in Rebecca Lilly's case, make the cells more sensitive to another treatment.

"We're very excited about this," said Cory Raffel, a neurosurgeon at the Mayo Clinic and chief of the pediatric brain tumor gene therapy test in which Becca is the first patient. "But it's absolutely experimental therapy. We're trying to see if it works. We don't know yet."

Such tests are first steps in "a long, long journey," said Stephen G. Marcus, vice president of Genetic Therapy Inc. (GTI), a biotechnology company in Gaithersburg. "What we're dealing with is a whole new concept of medicine." Gene therapy someday may allow doctors not merely to treat the results of a disease, as current cancer therapies do, but "to get at its root cause -- to get into the genetic material of a cell and fix it," Marcus said.

In Becca's case, the hope is that smuggling a gene into the tumor cells in her brain will make them cooperate -- under the influence of a follow-up drug -- in their own destruction.

That may sound simple, but finding a way to sneak the gene safely inside tumor cells is a huge challenge. It requires scientists to outsmart defenses the body has developed to protect itself against just such an invasion.

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irst, they needed what gene therapists call a vector -- a microscopic "taxicab" to deliver the gene into the cells. They hit on a Trojan horse approach, using some of nature's most ingenious spies: viruses.

"Viruses are very efficient and phenomenally brilliant at getting into cells and causing dreadful problems," Marcus said. The trick in gene therapy is to harness a virus's ability to invade a cell -- without letting it do any harm.

The taxicab that carried the crucial gene into Becca's brain was derived from a virus that was disabled in the lab so that it could no longer reproduce and cause disease. But it could still deliver its passenger, the gene, behind enemy lines. "It's like having a little spy in there that will sabotage the tumor cells," said the Mayo Clinic's Raffel.

Human brain cells stop reproducing at a very young age. That may be a dismaying thought to aging adults, but to gene therapists it signals a crucial advantage. The gene-toting virus injected into Becca's brain targets only dividing cells. Since cancer cells are the most rapidly dividing cells in the brain, the virus homes in on the tumor while leaving the normal brain alone.

"Will it get into enough tumor cells? That's a big question," said Roger J. Packer, director of the brain tumor program at Children's National Medical Center. The virus may not need to invade every tumor cell; studies suggest that when some tumor cells are killed, others nearby also die. Doctors call it "the bystander effect."

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fter studies in rats showed promise, the method was tried in 30 adult humans, beginning in 1992. The results have not yet been published, but scientists familiar with them say they were encouraging. In the cautious wording of the consent form the Lilly family signed: "Several patients showed a decrease in tumor size after this treatment."

"This is not the panacea, but it is the beginning, the first crude step," said Mitchel S. Berger, a neurosurgeon at the University of Washington who has treated adult brain cancer patients with gene therapy. "We've seen some fascinating results."

With approval from the National Institutes of Health and the Food and Drug Administration, the study in children will include 15 patients -- at Children's Hospital here, the Mayo Clinic in Minnesota, Children's Hospital in Los Angeles and the University of Washington in Seattle. The treatment costs between $25,000 and $50,000, including follow-up brain scans and blood work. The gene therapy part of Becca's treatment is paid for by GTI, which developed the technology and holds exclusive marketing rights.

Gene therapy is not "one big thing that either works or doesn't," GTI's Marcus said. It's an array of new attacks on diseases for which medicine has no good answer. Becca Lilly's therapy, for example, is not aimed at replacing a missing gene or fixing a genetic defect or changing her genetic makeup.

"We're not making a fat person thin," Berger said. "We're not doing anything to their personality or their appearance or their psyche or their soul. We're trying to make the tumor go away."