In a bold new form of treatment, government scientists last September injected a missing gene into a 4-year-old girl with a rare inherited disorder that left her without an immune system.

Called human gene therapy, this radical approach seeks to cure patients by replacing missing or defective genes with normal ones. Cells are taken from the patient, genetically altered in the laboratory, and then re-injected into the patient in hopes of changing the basic genetic structure of the body's cells.

Instead of treating symptoms of disease, this therapy corrects the genetic cause of the problem. Scientists expect that in the future, gene therapy may be used to reverse inherited illnesses such as cystic fibrosis that are caused by a single defect in the person's genetic makeup. Gene therapy may even play a role in fighting such common killers as cancer and heart disease that have a genetic component -- making this approach as important to medicine as the development of antibiotics, some researchers say.

The first experiment involved a child with a disease caused by a deficiency of adenosine deaminase, an enzyme that eliminates a toxin that kills certain white blood cells. Since she was born without a normal copy of the gene that makes the enzyme, she did not have a normal immune system. Most children with this disease die before age 2.

The team of W. French Anderson of the National Heart, Lung and Blood Institute and R. Michael Blaese and Kenneth Culver of the National Cancer Institute developed a way to put the missing gene into the white blood cells that make up her immune system.

In a preliminary report to the Food and Drug Administration last month, the NIH scientists said they were encouraged by the girl's progress. For the first time in her life, she was producing normal numbers of white blood cells. It will be at least a year before scientists know whether the treatment has cured her.

In 1991, two or three more patients with this rare disorder will be treated with gene therapy. In addition, NIH researchers will use this technique to treat patients with advanced melanoma, an often fatal form of skin cancer. Scientists plan to insert a gene into white blood cells that will make a protein called tumor necrosis factor, which kills cancer cells.

While the potential of gene therapy has generated much excitement in the scientific community, its application is currently limited to a small number of patients because gene-transfer techniques are so complex and labor-intensive.

"The difficulties of delivering genes to cells is still a nagging problem," said Philip Leder, genetics chairman at Harvard Medical School.

Alternative approaches are under development at several research centers. James M. Wilson at the University of Michigan and George Wu at the University of Connecticut have found a way to link genes to proteins so that the protein acts like a guided missile to carry the gene to a specific cell within the body. In animal experiments, they have used the carrier system to insert genes into liver cells via the blood.