Adeno-associated virus (AAV) is a small (20 nm in diameters) replication-defective and nonenveloped virus that infects humans and some other primate species. AAV is not presently known to result in disease and consequently the virus causes a very mild immune response. Recombinant AAV vectors can efficiently transfect multiple cell types, including quiescent and dividing cells, and induce persistent gene expression in vivo without integrating into host genome and causing any disease.