Gene therapy is the introduction of a gene into cells to reverse a functional defect caused by a defect in a 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 (deoxyribonucleic acid, DNA; 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...