Gene therapy may be defined in broad general terms as follows: Introduction of a normal functional gene into cells, which contain the defective allele of the concerned gene with the objective of correcting a genetic disorder or an acquired disorder.

Application of gene therapy involves the following basic developments:
(i) identification of the gene that plays the key role in the development of a disorder,
(ii) determination of the role of its product in health and disease,
(iii) isolation and cloning of the gene, and (iv) development of an approach for gene therapy. The candidate disorders for gene therapy are selected on the basis of the following criteria:

(i) the disease should be life threatening,
(ii) the gene responsible for the disease has been cloned,
(iii) a precise regulation of the gene should not be required, and
(iv) a suitable delivery system should be available.