WASHINGTON — A 4-year-old girl was being prepared to become the first patient to receive gene therapy in a study that scientists say could revolutionize the treatment of disease, officials said Thursday.

Researchers at the National Institutes of Health said they expected to be ready to begin the gene-restoration treatment Friday on the child, who was born with a defective immune system unable to fight infections and diseases.

If the treatment is successful, it would put researchers on a new frontier in medicine that could lead to similar treatments for cancer, hemophilia, sickle cell anemia, AIDS and other diseases.

“We feel that this is potentially a major new therapy option that should have significant clinical effects in the next century,” said Dr. W. French Anderson, a researcher at the National Heart, Lung and Blood Institute at NIH.

The child, who was not identified, has a rare genetic disease called ADA deficiency — a defective gene which means her immune system does not function adequately because the enzyme it produces is defective. This leaves her vulnerable to infections and diseases most people can fight off.

Researchers have already begun the initial work necessary for the first patient’s treatment. Weeks ago, they took white cells from some of her blood and began growing them in tissue culture. Then they inserted a normal ADA gene into these cells with a genetically engineered virus. The virus has been disabled and is no longer infectious, but serves as a vehicle to get the normal gene into the cells where it produces the required enzyme.

Once a sufficient number of ADA-corrected cells have been grown, they are infused into the patient intravenously.

Only about 10 children a year are born with the disease. Without treatment they usually die within the first two years of life.

The best current treatment is a bone marrow transplant with a donor whose tissue type matches the child’s, such as from a brother or sister. Experts say most children do not have a matched donor.

For the children without a match, their options are a partially matched bone marrow transplant, which fails half the time, or enzyme replacement therapy with a drug called PEG-ADA. Weekly injections of the new drug decreases the severity of infections for some patients, but it does not fully restore their immune systems.