Gene therapy is an experimental treatment in which normal genes are introduced into the body’s cells to correct or modify the cell’s function. This technique was first employed successfully in 1990 when it was used to treat severe combined immunodeficiency (SCID), a disease resulting from a deficiency of the enzyme adenosine deaminase (Anderson 1995). This enzyme normally converts a molecule called deoxyadenosine into another molecule called inosine. A person who inherits two defective alleles of the gene responsible for adenosine deaminase production cannot convert deoxyadenosine into inosine. This leads to the rapid accumulation of deoxyadenosine, which is then converted into a toxic product that kills white blood cells involved in fighting infections, resulting in an almost complete failure of the immune system and early death. To treat the disease, researchers removed white blood cells from affected patients, inserted a normal copy of the adenosine deaminase gene into those cells, and reintroduced the modified cells into the body’s circulation. The modified cells then could produce sufficient adenosine deaminase to prevent deoxyadenosine accumulation. This kind of gene therapy is called somatic gene
