Gene therapy is a treatment that involves altering the genes inside your body's cells to stop disease.Genes contain your DNA - the code that controls much of your body's form and function, from making you grow taller to regulating your body systems. Genes that don't work properly can cause disease.Gene therapy replaces a faulty gene or adds a new gene in an attempt to cure disease or improve your body's ability to fight disease. Gene therapy holds promise for treating a wide range of diseases, including cancer, cystic fibrosis, heart disease, diabetes, hemophilia and AIDS.Researchers are still studying how and when to use gene therapy. Currently, in the United States, gene therapy is available only as part of a clinical trial.