If scientists can successfully use gene therapy to treat a patient with ADA, the door will be open to treat some of the 3,000 other genetic diseases.

But this idea does not mean gene therapy "will have an enormous impact on the practice of medicine in the near future," says Dr. W. French Anderson, head of molecular hematology laboratory at the National Heart, Lung and Blood Institute.

"The first gene therapy patients will be studied very carefully over a period of time -- monthas and years -- to be sure things don't go wrong in this really revolutionary, new type of therapy. But if, in fact, the first patients are helped and there are no unexpected safety problems, it is very likely . . . that by a decade from now, a large number of genetic diseases will be treated by gene therapy."

Before any disease can be treated with gene therapy, researchers must have identified the exact genetic defect responsible and cloned a gene to correct it. The ailment must be a disease that can be correct through genetic manipulation of the blood or bone marrow -- easily accessible for treatment. Bone marrow contains the stem cells, which mature and squeeze through bone to become the rich array of cells that course through the bloodstream. Many of them become red blood cells and lymphocytes, which are white blood cells important for fighting infection. By altering the genes of these immature cells in the bone marrow, researchers have found the ideal conditions for producing normal cells in the blood. Diseases that meet these qualifications include cystic fibrosis, sickle cell anemia, thalessemia, Tay-Sachs, hemophilia, PKU and maple syrup urine disease.

Until last year, scientists also thought that Lesch-Nyhan would be among the group of genetic diseases treated with early gene therapy. But all that changed when standard bone marrow transplants failed to produce any improvement. "It looks like in Lesch-Nyhan, the genetic defect produces irreversible brain damage by birth," Anderson said. "So it's untreatable by gene therapy ."

Genetic diseases involving defects in the organs, such as the kidney, liver heart or brain, or in the nervouse system also will have to wait for more scientific advances. "After the first generation diseases, a lot more is going to have to be learned about gene regulation,"' says Dr. Philip Kantoff, a medical fellow working with Anderson at NHLBI. "To get to the third step, more is going to have to be known about molecular genetics and molecular biology."