The transfer of GENES into a patient to combat the effects of their own abnormal genes which are causing disease(s). The GENETIC ENGINEERING technique used is SOMATIC cell gene therapy in which the healthy gene is put into body cells that are not part of the normal reproductive process but can divide to produce other cells – for example, STEM CELLS that develop into BONE MARROW. Descendants of these altered cells will be normal and, if sufficient numbers are produced, the patient's genetic disorder should be remedied. The abnormal gene, however, will still be present in the treated individual's GERM CELLS (eggs or sperm) so he or she can still pass the inherited defect on to succeeding generations.

Other potential problems include that the introduced gene may not last; the recipient may mount a severe immune reaction to it; the virus which helps introduce the gene may itself cause problems; and most genetic disorders are not the result of just one faulty gene acting alone. The first gene to be authorised for sale by the European Medicines Agency (in 2012) is tiparvovec (marketed as Glybera®) intended to treat lipoprotein lipase deficiency, an extremely rare genetic disorder, which can lead to bouts of severe pancreatitis. The therapy relies on a virus to deliver functional copies of the lipoprotein lipase gene to the body's muscle cells. US Food & Drug Administration has not yet licensed any human gene therapy product for sale. Numerous trials are in progress for candidate gene therapy, for example for cystic fibrosis