Research over the past 20 years has focused on the development of adenoassociated virus (AAV) DNA delivery vectors for the treatment of several hereditary human diseases. Among viral vectors for gene therapy approaches, AAV has several advantages:(i) the majority of transgenic DNA persists as episomes rather than relying on host chromosome integration,(ii) AAV is considered nonpathogenic, and (iii) natural AAV serotypes transduce most nondividing and dividing cell types. 1 Furthermore, recent advancements in the understanding of functional capsid regions, as well as random capsid sequence selection techniques, demonstrate the ability to engineer AAV vectors for enhanced transduction of specific tissues as well as an enhanced ability to evade the immune system. 2–4

However, despite these desirable attributes, AAV vectors suffer from a major limitation for the treatment of disorders requiring large-gene transfer: the~ 26-nmdiameter virus capsid packages a singlestrand DNA payload of~ 5 kb. However, this packaging dogma was recently challenged by a report of the unique ability of AAV serotype 5 (AAV5) capsids to package genomes of≈ 9 kb, which mediated successful