The fact that genes can be cloned to several thousand copies through genetic engineering has given rise to an entirely novel model of therapeutic device viz., Gene therapy. Gene therapy involves the replacement of corrective genes in place of defective genes in human. There are two types of gene therapy. They are 1). Somatic cell gene therapy and 2). Germ line cell gene therapy. Both may be employed for treating the inherited diseases. In somatic cell gene therapy the patient's cells are taken from blood or bone marrow, or skin and brain, treated through genetic engineering and then replaced to the patient. Somatic cells are non-reproductive cells. These cells and their genes when corrected, the genetic changes will not be inherited to their children. On the other hand, this type of treatment could cure diseases caused by single gene mutations.In this therapy, retroviruses are employed as carriers of the genes. Their genetic material, RNA is copied into DNA. To produce effective vectors certain essential genes are spliced out from the viruses. This will render the virus harmless and allow space for the therapeutic genes to be inserted. Hundreds of millions of copies of the genes are needed for somatic cell gene therapy. To carry them, the vectors should also be multiplied in equal numbers. Experimentally, such enormous copies of virus vectors are made using helper virus. The integrated vector and the cloned gene is used in somatic cell gene therapy. Somatic cell gene therapy has been successful in animals, so far.
The gene therapy is extended to reproductive or germ line cells in order to prevent the genetic defects being inherited to children. The technique has been shown to work positively in animals. In this, DNA is injected into the nuclei of single celled embryos of cows, mice and sheep. The DNA may get integrated into the chromosomes. The intergrated DNA, ie., the new genes can direct the synthesis of new proteins into animals which develop from these embryos. Germ line therapy has already been incor-porated developed in human embryos. Genes are inherited likewise through retroviral vectors into human germ cells. Germ line therapy in human is feasible, at any time, since eggs, sperms and single celled embryos can be collected and stored by deep freezing using liquid nitrogen. In vitro fertilization also may favour the above therapy. However, the therapy is still underway.