What is gene therapy? Illustrate using the example of adenosine deaminase (ADA) deficiency.

Gene therapy is the collection of methods that allows correction of gene defect that is diagnosed in a child or embryo. Adenosine deaminase gene is deleted from this gene disorder and it can be cured marrow transplantation or by enzyme replacement therapy but since cells are not immortal so this not complete curative. Lymphocytes of the patient are grown in a culture outside the body. Genetically engineered lymphocytes need to be periodically infused but if gene isolated from marrow cells is been changed to producing ADA cells at early embryonic stage it could be permanently cured.