A member of the parvovirus family that is used as the basis for vectors in certain gene therapies. A recombinant AAV particle lacking viral DNA is now favoured for delivering DNA for therapeutic applications. The DNA of interest is inserted between a suitable promoter and terminator, and all three sequences are flanked by inverted terminal repeat (ITR) sequences, needed for packaging in the virus particle and replication following entry into the host cell. There the DNA persists as an independently replicating episome in the nucleus. The engineered virus contains no viral genes, so the likelihood of an immune response by the host is reduced. Also, the protein coat and promoters can be designed so that the virus targets specific tissues in which to deliver its DNA. Naturally occurring AAVs require a ‘helper’ virus, such as an adenovirus, to replicate. They are harmless to humans and stimulate little or no immune response, so that infected cells are not destroyed and the treatment is potentially long lasting. Also, they can infect both actively dividing and nondividing cells, enabling them to target a wide range of cell types, including the central nervous system, eye, and muscles. However, the small size of the viral genome restricts the size of the inserted gene to about 5 kbp. Patients with Parkinson’s disease have been injected with AAVs into the brain to deliver a gene encoding glutamate decarboxylase, the enzyme that catalyses synthesis of the deficient neurotransmitter gamma-aminobutyric acid.