Adeno-associated viruses (AAVs) are small, icosahedral viral particles measuring 20–25 nm in diameter. They are so called because they can multiply only in cells simultaneously infected with adenovirus, but cannot multiply independently because they lack enough DNA. These viruses have been classified in the genus Dependovirus. Four serotypes (1–4) have been recog-nized, of which types 1, 2, and 3 are of human origin and type 4 is of simian origin. The pathogenic role of the human sero-types of AAVs is not clear.