The first human gene therapy experiment, in which researchers are trying to repair the defective immune system of a 4-year-old girl, seems to be working, scientists told a Food and Drug Administration review panel last week.

After three months of treatment, the girl's immune system has become virtually normal for the first time in her life, said R. Michael Blaese of the National Cancer Institute (NCI), one of the gene researchers. And, so far, there have been no harmful side effects from the treatment.

"These are just the first preliminary results, but we are certainly excited about them," said W. French Anderson of the National Heart, Lung and Blood Institute, a leader in gene therapy research.

Although the child's disorder is rare, human gene therapy, in which new genes are inserted in the cells of human patients to perform functions missing in the patient's genes, is expected one day to be a major new therapy for illnesses as common as heart disease, cancer and even AIDS.

The girl, whose family has not permitted release of her name, will continue receiving infusions of genetically altered cells for another three months. Then researchers will conduct a comprehensive evaluation of her immune system, comparing it with how well her defenses worked before the experiment began.

The child was born with a defective gene that prevented her body from making an enzyme known as ADA. Without the enzyme, a toxin built up in her blood, killing her T cells, a type of white blood cell that plays a central role in orchestrating the body's defenses against infections. This is the same cell type that is destroyed in AIDS.

To correct the deficiency, the National Institutes of Health team removed some of her white blood cells and grew them in the laboratory. They then exposed the cells to a virus that had been designed to carry a normal copy of her defective gene.

When the virus infected her blood cells, it used its usual mechanisms to splice the new gene into the cells' chromosomes. This gives the cells the ability to make the normal enzyme and get rid of the toxin. With the toxin eliminated, the researchers expected her immune system cells to grow normally.

So far, that seems to be happening.

Preliminary evidence suggests that the gene-treated cells are producing the missing enzyme in her body. Kenneth W. Culver, an NCI researcher and member of the gene therapy team, has isolated white blood cells from her body that produce normal amounts of ADA. It also appears that the new supply of ADA is stimulating the girl's untreated white blood cells to grow more normally, since the number of such cells has risen higher than the number that were injected. This part of the experiment is difficult to evaluate, however, because the girl continues to receive injections of a synthetic form of ADA that she had been getting before the experiment began.

Whether the gene therapy treatment will restore normal functioning to the girl's immune system has yet to be proved, but there are hints that she is now able to fight off bacteria and viruses as they enter her body.

"She has had only one cold {since treatment started}," Blaese said last Thursday. Her parents are excited about this, he said, because her weakened immune system had left her prey to every passing virus and she had been constantly ill.

"But," Blaese cautioned, "there is no way to know that that is because the treatment is working. It is encouraging; everything is very encouraging.