Gene therapy involves the introduction of a gene into cells in order to reverse a functional defect in the host genome (the set of genes present in an organism).

The use of viruses quickly became an attractive possibility once the possibility of gene therapy became apparent. Viruses require other cells for their replication. Indeed, an essential feature of a virus replication cycle is the transfer of their genetic material--DNA (deoxyribonucleic acid) or ribonucleic acid (RNA)--into the host cell, and the replication of that material in the host cell. By incorporating other DNA or RNA into the virus genome, the virus then becomes a vector for the transmission of that additional genetic material. Finally, if the inserted genetic material is the same as a sequence in the host cell that is defective, then the expression of the inserted gene...