Gene therapy burst into the news last fall, when a teenager from Tucson died in an experiment at the University of Pennsylvania. But while politicians and the press have spent the year focusing on accusations of lax government oversight and shoddily run clinical trials, the science of gene therapy has been making quiet, steady progress.

Little by little, researchers say, they are finding new and better ways to deliver genes to their target cells. Clinical trials of gene-based treatments for hemophilia are showing promise, and certain cancer patients appear to respond to gene therapy. And the field got a big ego boost this spring when French scientists reported they had used gene therapy successfully to treat babies born with defective immune systems.

And so it was with mixed emotions -- worry and regret, enthusiasm and a little bit of defiance -- that 2,500 gene therapy scientists from around the nation, and the world, gathered here this week for the third annual conference of the American Society of Gene Therapy. It has been, they agreed, a roller coaster of a year.

''It's finally coming together,'' said Dr. Savio L. C. Woo, a soft-spoken molecular biologist who, as the society's president, found himself testifying in Congress on more than one occasion in the past several months. ''We are finally seeing this glimpse of hope that this new technology is going to bear fruit in the clinic. And yet we have had this serious setback.''

The setback, of course, was the death of 18-year-old Jesse Gelsinger, who suffered from a metabolic disorder and had volunteered for an experiment to test gene therapy for babies with a fatal form of the disease. His presence was acutely felt at the conference. In his presidential address, Dr. Woo asked those attending to stand in a moment of silence for ''the young man who has given his life in pursuit of an ideal treatment'' and ''to assure him in spirit that the scientific community is galvanized to do our very best to help fulfill his dream one day.''

Mr. Gelsinger's death has had ripple effects throughout the field. It touched off a discussion of financial conflict of interest in gene therapy experiments, prompting the society to issue guidelines barring members from running clinical trials if they had a financial stake in the companies sponsoring their studies.

It has also caused a slowdown in human experiments; an official from the Food and Drug Administration, which recently issued more stringent regulations to govern the conduct of gene therapy clinical trials, said here that requests to test gene therapy in people had dropped off sharply in recent months.

''The field is in transition,'' said [Dr. Robert Wallace Malone (born 1959)], a researcher at the University of Maryland. ''I think it is transitioning to a more sober, realistic recognition of what is achievable. I believe there is a new humility in the field.''

Dr. Rajendra Kumar-Singh is typical. At 32, Dr. Kumar-Singh is an assistant professor of ophthalmology at the University of Utah and is just starting his career in gene therapy. He is studying treatments for retinitis pigmentosa, an inherited condition that causes blindness. By administering an infusion of gene-altered viruses to baby mice, he said, he has staved off blindness for 10 weeks in animals that would otherwise have lost their sight within 17 days of birth.

But Dr. Kumar-Singh said he would be cautious before testing the therapy in people. ''Perhaps we were moving too fast,'' he acknowledged, echoing the sentiments of some critics, including [Dr. Harold Eliot Varmus (born 1939)], the former director of the National Institutes of Health, who felt gene therapy researchers moved too quickly into clinical trials. Still, Dr. Kumar-Singh is optimistic about gene therapy's future. ''We are seeing a revolution in medicine right now,'' he said, ''and gene therapy is at the forefront of it.''

The idea behind gene therapy is disarmingly simple: to treat or cure disease by giving healthy genes to patients with defective ones. But in the 10 years since the first human experiment was conducted by researchers at the National Institutes of Health in Bethesda, Md., the results have been largely disappointing.

One reason is that scientists have had trouble devising delivery vehicles, called vectors, that can direct genes into the proper cells and get them to function once they are there. Vectors are typically made by inserting genes into deactivated viruses that target certain cells, literally infecting them with healthy DNA.

Mr. Gelsinger's death, however, raised safety questions about one of the most commonly used viruses, adenovirus, which causes the common cold. In most patients, adenovirus produces mild, flulike symptoms. But in Mr. Gelsinger, it provoked a fatal immune response.

Even before Mr. Gelsinger's death, molecular biologists had been turning their attention to a different virus, the adeno-associated virus, or AAV, which is thought to be safer than adenovirus. Now, a team of researchers at Children's Hospital of Philadelphia, Stanford University and Avigen Inc., a biotech company, is reporting promising results in hemophilia patients who received a genetically engineered form of AAV that contains the gene for production of Factor IX, a protein that is needed to make blood clot.

The team, led by Dr. Katherine High of the Philadelphia children's hospital, began a small safety study after they found the treatment could essentially cure dogs of hemophilia. So far, six patients have been enrolled. The first three received the gene therapy at a dose so low it was not effective in the dogs. But to the scientists' surprise, the patients began expressing minute amounts of Factor IX -- enough that it improved their conditions and reduced their need for the standard hemophilia treatment, injections of Factor IX.

''We were delighted, but skeptical,'' said Dr. Catherine S. Manno, who is running the clinical trial. ''Only after repeated measurements over a period of months did we become convinced that these levels were real.''

Although the hemophilia experiments are still in their early stages, leaders in gene therapy say that, aside from the French work, Dr. High's research is the most exciting in the field. Dr. Donald B. Kohn, an immunologist at Children's Hospital of Los Angeles, said, ''Hemophilia may be within a shot of being cured by this approach.''

The word cure -- the ''C-word,'' as [Dr. Robert Michael Blaese (born 1939)], who performed the first human gene therapy experiment in 1990, calls it -- is one that gene therapy researchers have learned to use with caution.

''We will not talk about cure,'' said Dr. Alain Fischer of the Necker children's hospital in Paris. ''Cure means forever.'' Yet Dr. Fischer's work on babies with a form of severe combined immune deficiency, or SCID (pronounced like skid), is the closest thing gene therapy has seen to a cure. His study provided gene therapy advocates with what they have long lacked: proof, in principle, that the concept can work.

Dr. Fischer's findings, published in April in the journal Science, were a popular topic among the scientists in in Denver. ''The field is now an established principle in medicine,'' said Dr. Theodore Friedmann, a professor of pediatrics at the University of California at San Diego who said he had been pursuing gene therapy since 1968. ''That's the story -- a brand new concept in biomedicine, irretrievable, and it's beginning to work.''

In the Science article, Dr. Fischer recounted the successful treatment of two babies, both of whom had normal immune systems 10 months after receiving gene therapy. In Denver, he told reporters that he had since treated three more children. Of the five, four have had ''a complete or near complete recovery'' of their immune systems, he said. The outlook for the fifth, who had severe complications from infection at the time of treatment, is less certain.

Experts say one reason Dr. Fischer was successful where others had failed is that SCID is particularly suited for gene therapy. The first human gene therapy experiment, conducted by Dr. Blaese and [Dr. William French Anderson (born 1936)], was directed at curing adenosine deaminase, or ADA, deficiency, another form of SCID. But it has been difficult to gauge that study's outcome because a drug, PEG-ADA, is available to children with the disease, and scientists consider it unethical to withdraw the medicine.

In Denver, however, an Italian researcher, Dr. Claudio Bordignon, announced that he had solved that problem by being ''lucky to find a patient who was unlucky'' -- a child who does not respond well to PEG-ADA. The patient, now 5, was given her first infusion of corrective genes in 1996 and was slowly weaned from the drug. She has not taken PEG-ADA for one year now, and her immune system is functioning better now than before, Dr. Bordignon said.

He theorized that the drug might have somehow suppressed the effects of the gene therapy. ''After discontinuation of PEG-ADA,'' he said, ''all the genetically engineered cells have come out.''

Whether, or when, gene therapy will ever become a part of mainstream medicine remains a matter of debate. Most gene therapy experiments are still being conducted in animals, and those being tested in people are producing mixed results.

At the Denver conference, for instance, scientists from [Vical Incorporated], a San Diego company, reported preliminary results from 52 patients who have been enrolled in a 70-patient study of gene therapy for advanced skin cancer. In the study, a gene that alerts the immune system to recognize and kill foreign tissue is administered directly into the patients' tumors.

According to Dr. Deirdre Y. Gillespie, Vical's chief operating officer, 10 percent of patients responded extremely well to the therapy, with their tumors shrinking in size by 50 percent or more. In another 15 percent of patients, the therapy stopped the progression of disease, and the therapy reduced the size of tumors in some of those patients as well.

Those may not sound like spectacular results, but as Dr. Gillespie noted, there is currently no effective treatment for advanced skin cancer, and the patients in the study had failed all other therapies. In a sense then, the cancer study encapsulates the state of gene therapy as a whole.

Dr. Blaese put it this way: ''For the average person, the progress may look to be minor. But you need to put the developments of this field in context. We just started 10 years ago.''