The functional gene is introduced in addition to the defective gene endogeneous to the cell(s) in this somatic cell gene therapy, i.e. the improved cells consist both the defective (endogeneous) and the normal (introduced) copies of the gene. There are two basic approaches to augmentation therapy. First was used in the first two patients on whom gene therapy was attempted to right the genetic disorder known Severe Combined Immune Deficiency (SCID) syndrome formed by adenosine deaminase (ADA) deficiency. (i) General ADA gene copies were formed by cloning and then (ii) packed into a faulted retrovirus; mostly viral genes were replaced by the ADA gene. (iii) Lymphocytes were linked from the patients, and (IV) the recombinant retroviruses were used to infect the lymphocytes. And (v) the infected cells showing the ADA gene were injected back into the patients. The usual ADA gene was expressed in the patients, and ADA deficiency was partially right, this resulted in an improvement in the patient's immune system. Different kinds of viral vectors have been used to deliver genes into target/stem cells, like lymphocytes, cultured in vitro, bone marrow cells. The target cells themselves are obtained from the concerned patient (more desirable) or from a matched donor. The reservations about security of retroviral vectors is sought to be resolved by developing suicide vectors which cannot replicate after delivery of the gene. The other major problems of this approach are: (i) low frequency of transfect ion of stem cells, (ii) persist ability of the integrated gene, (iii) duration of the gene expression, (iv) lack of suitable regulation of gene expression etc. More recently, interest has targeted on physical methods of gene delivery like Ca2+ phosphate co precipitation, particle gun, electro oration etc. The next approach is the direct injection of DNA into the tissues either as protein complexes (in order to bring about receptor-mediated transfer of DNA into a particular tissue, for example liver) or even as naked DNA into skin or muscle. Interestingly these cells obtain the DNA and show the gene product. Exciting results have been obtained with experimental familial hypercholesterolemia, where LDL receptor stage has been augmented by insertion of the gene as a sialoglycoprotein complex. The problems in this approach relate to the frequency of cells taking up and showing the gene and, more specifically, the duration of expression. The gene delivery methods used for gene therapy may also be used for healing of cancer or AIDS. In the case of cancer, a toxin encoding gene can be delivered into the cancer cells. In the same way, appropriate interleukin genes can be delivered to boost up the body's defence mechanisms (in case of AIDS).