Gene therapy is defined as the introduction of normal genes in place of defective genes in the cells to correct genetic disorders.

The gene therapy is of two types:

1. Somatic or non-inheritable gene therapy involving delivery of a normal gene into the individual’s somatic cell. Some of the patient’s own cells are removed, repaired and re-implanted in the patient.
2. Germ-line or inheritable gene therapy involving delivery of a normal gene into the embryo.

Gene therapy has been employed in diagnosing ADA deficiency, cystic fibrosis, haemophilia, AIDS. Tay-Sach’s disease, asthma, brain tumour, melanoma, cancers of
breast, ovaries, prostate, liver and colon, muscular dystrophy, neurological disorders, diabetes, cardiovascular diseases, etc.

The first clinical gene therapy was given in 1990 to a four-year old girl Ashanthi DeSilva who was suffering from the rare genetic disorder adenosine deaminase-deficient severe combined immunodeficiency disease (ADA-SCID).