The treatment, which was first tested in humans in 1990, can be performed inside or outside of the body. When it’s done inside the body, doctors may inject the virus carrying the gene in question directly into the part of the body that has defective cells. This is useful when only certain populations of cells need to be “fixed.” For example, researchers are using it to try to treat Parkinson's disease, because only part of the brain must be targeted. This approach is also being used to treat eye diseases and hemophilia, an inherited disease that leads to a high risk for excess bleeding, even from minor cuts.