A new, and potentially very beneficial, application of DNA technology is gene therapy. The goal in gene therapy is to correct genetic diseases caused by mutant genes that do not produce functional enzymes. In gene therapy, the normal gene is inserted into cells to restore proper enzyme production. In some cases (ex vivo), cells are removed from a patient’s body, the proper DNA is inserted, and the cells are then returned to the patient’s body. In other cases (in vivo) carrier molecules or agents, such as viruses, are used to take the normal gene into cells in the patient’s body.