Adeno-associated virus (AAV) is a small virus which infects humans and some other primate species. AAV is not currently known to cause disease and consequently the virus causes a very mild immune response. AAV belongs to the genus Dependovirus, which in turn belongs to the family Parvoviridae. The virus is a small (20 nm) replication-defective, nonenveloped virus. AAV4 is one of 11 known AAV serotypes identified to date.