An example of gene therapy is

On September 14, 1990 researchers at the U.S. National Institutes of Health performed the first (approved) gene therapy procedure on four-year old Ashanti DeSilva. Born with a rare genetic disease called severe combined immune deficiency (SCID), she lacked a healthy immune system, and was vulnerable to every passing germ. Children with this illness usually develop overwhelming infections and rarely survive to adulthood. In this gene therapy procedure, doctors removed white blood cells from the child's body, let the cells grow in the lab, inserted the missing gene into the cells, and then infused the genetically modified blood cells back into the patient's blood stream. Laboratory test have shown that the therapy strengthened her immune system.