The main problem here is that the frequency with which the normal gene copy goes where it is designed to go is extremely low. Both ways need tools for delivering the genes. These tools are the vectors, the vehicles of gene delivery and the Achilles heel of gene therapy. Most gene therapy has involved the use of viruses as carriers of the gene. This has proved to be the most efficient method to date, performing many of the tasks necessary to achieve successful gene transfer, such as binding to a target cell and delivering the viral genome to the nucleus in a stable way.
Retroviruses are the most used viral vectors.
Retroviruses can induce insertional mutagenesis as well as potentially transforming these cells. Adenoviruses, which are a family of DNA viruses that can infect both dividing and non-dividing cells, causing benign respiratory tract infections in humans, are the second most used vectors for gene delivery. Promising viral vectors are adenoassociated virus and lentivirus. Adenoassociated virus is a simple, non-pathogenic, single stranded DNA virus. This virus infects a wide range of cells, including lung and muscle cells, and it integrates its genes within the host's.