All the cells present in the body contain genes and an approach targeted to modulate the expression of genes is known as gene therapy. This novel approach has emerged by the end of 20^th century and comes up with a promise of treating a large number of genetic disorders by ultimately modifying the gene expression (Jin et al, 2008). The two forms of gene therapy are somatic and germ line.

Somatic gene therapy: During this therapeutic approach, the genetic diseases are treated by inserting recombinant genes into non-reproductive or somatic cells. This kind of approach is limited to the patient and does not get transmitted to the future generations of. A normal copy of the malfunctioning gene is inserted into the mutant cells due to which an improvement in the disease phenotype is observed (Dorin, 1996). In the recent past, this technique has been found to be successful in case of familial hypercholesterolaemia and adenosine deaminase deficiency. For treating these disorders hepatocytes and T cells have been used for gene transfer respectively (Dorin, 1996). It has also shown significant success in case of cystic fibrosis and cancer. Various vehicles, like viral and non viral vectors help in the introduction of the genetic material into the mutated cells (Elias and Annas, N.D). This therapeutic approach is categorized into three sections:

• ex vivo - The genetic engineering of the cell is done outside the body and finally the cells are returned to the body. (Ex: blood cells)
• in situ - The cells of the affected tissues are directly transfected with vectors carrying genetic material. (Ex. Tissues of the respiratory tract)
• in vivo - The vector is directly injected into the blood stream and this method is still in the developmental stages (Anderson, N.D).

However this approach produces short term results as the tissues die and get replaced with other cells.