Adeno-associate viruses (AAV) are defective, only undergoing productive replication in cells that are co-infected with another DNA virus, most commonly an adenovirus, which modifies the host environment in multiple ways that are essential to enable the AAV life cycle; by themselves AAVs are not associated with any known pathology. Under certain conditions, for example in the presence of DNA damaging agents, limited AAV replication can also be detected in the absence of helper viruses. Co-infection with helper virus triggers a lytic cycle as do some agents which appropriately perturb host cells. Wild-type AAV integrates preferentially into human chromosome 19q13.3-qter; recombinant vectors lose this specificity and appear to integrate randomly, thereby posing a theoretical risk of insertional mutagenesis.