Adeno-Associated virus (AAV) is a non-pathogenic helper dependant parvovirus. AAV is a 4.7-kilobase, single-stranded DNA virus that contains two genes. Rep encodes four proteins necessary for genome replication (Rep78, Rep68, Rep52 and Rep40), whereas cap expresses three structural proteins (VP1-3) that assemble to form the viral capsid. AAV depends upon a helper virus such as an adenovirus for active replication and in the absence of a helper establishes a latent state in which its genome is maintained episomally or integrated into the host genome. A number of homologous human and nonhuman primate AAV serotypes have been identified; of these, AAV2 is the best characterized as a gene delivery vehicle and the only one that has been used in clinical trials.