Adeno-associated virus (AAV) is a single-stranded DNA virus that commonly infects humans without pathology. Therefore, AAV can be engineered to deliver DNA to target cells as a vector for gene therapy. There are several serotypes including AAV type 1, 2, 5, 6, 8, 9, DJ and rh.1. Among them, AAV2 is the best characterized and most commonly used. Cap gene of AAV gives rise to three capsid proteins, VP1, VP2 and VP3, with molecular weight of 87, 72 and 62 kDa, respectively. These capsid proteins assemble into a near-spherical protein shell of 60 subunits for AAV.