Introduction of required genes into cells and tissues to treat diseases / by delivery of normal gene to take over the function of tron-functional gene/by gene therapy, First gene therapy was given to four year old girl with Adenosine deaminase deficiency. 

Detailed Answer:

Gene therapy is an attempt to deal with hereditary or genetic or congenital diseases. This aims at correction of a genetic defect by delivery of a normal gene into an individual or embryo to take over or compensate the function for a non-functional gene. The first disease to have a gene therapy is ADA (Adenosine deaminase) deficiency. In this, the gene coding for enzyme ADA gets deleted leading to deficiency of ADA and problems in immune system. Gene therapy for ADA deficiency includes Isolation of lymphocytes from patient's blood and culturing them in-vitro.

Functional ADA cDNA are then introduced into the cultured lymphocytes. These lymphocytes are returned back to the patient's body. 

Permanent cure for this problem is the introduction of gene isolated from bone marrow cells producing ADA into cells at early embryonic stages.