Gene therapy is the insertion of genes into an individual's cells and tissues to treat a disease, such as a hereditary disease in which a deleterious mutant allele is replaced with a functional one. Although the technology is still in its infancy, it has been used with some success. Antisense therapy is not strictly a form of gene therapy, but is a genetically-mediated therapy and is often considered together with other methods.

Gene therapy using an Adenovirus vector. A new gene is inserted into an adenovirus vector, which is used to introduce the modified DNA into a human cell. If the treatment is successful, the new gene will make a functional protein.

In Ashanthi's gene therapy procedure, doctors removed white blood cells from the child's body, let the cells grow in the laboratory, inserted the missing gene into the cells, and then infused the genetically modified blood cells back into the patient's bloodstream. Whether the therapy strengthened Ashanthi's immune system is unclear, because the study did not have a control arm and she continued to receive standard therapy for her condition during the study. Moreover, the white blood cells treated genetically only work for a few months, after which the process must be repeated (VII, Thompson [First] 1993). As of early 2007, she was still in good health, and she was attending college. Some would state that the study is of great importance despite its indefinite results, if only because it demonstrated that gene therapy could be practically attempted without adverse consequences.

The road to the first approved gene therapy procedure was rocky and fraught with controversy. The biology of human gene therapy is very complex, and there are many techniques that still need to be developed and diseases that need to be understood more fully before gene therapy can be used appropriately. The public policy debate surrounding the possible use of genetically engineered material in human subjects has been equally complex. Major participants in the debate have come from the fields of biology, government, law, medicine, philosophy, politics, and religion, each bringing different views to the discussion.

Scientists took the logical step of trying to introduce genes straight into human cells, focusing on diseases caused by single-gene defects, such as cystic fibrosis, hemophilia, muscular dystrophy and sickle cell anemia. However, this has been much harder than modifying simple bacteria, primarily because of the problems involved in carrying large sections of DNA and delivering them to the correct site on the comparatively large human genome. Today, most gene therapy studies are aimed at cancer.