AAV and rAAV
Adeno-associated virus (AAV) is a small, single-stranded DNA virus which replicates only in the presence of a helper virus, such as Adenovirus (Fig. 1, AAV 3D structure). Although more than 80% of the world population carries AAV, no disease is directly connected with it. With the help of the Rep protein, wild-type AAV can integrate its genome into human chromosome 19q13.4 (or AAVS1) but results in no adverse events. The genome of engineered recombinant AAV (rAAV) vectors contains no viral gene. However, it does contain inverted terminal repeats (ITR) as its packaging signal, which makes it impossible for the virus to self-replicate. The viral genes of AAV are separately supplemented for both producing rAAV capsid protein and packaging function during production. The “gutless” (95% sequence deleted in wild type AAV) rAAV has been evaluated by the FDA as a safe vector for use in human gene therapy (Fig. 2, rAAV vector).

Advantage of rAAV
The use of rAAV can mediate stable and long-term transgene expression in most mammalian tissues without pathogenicity and insertion in the host genome, which makes rAAV the most promising vector for gene therapy. rAAV has been widely used as a high-efficiency gene transfer vehicle in both basic research and clinical gene therapy during the past two decades. Now, numerous AAV serotypes are discovered or generated, which provides diverse choices for either broad or specific organ transduction (Fig. 3 presents AAV9 carrying EGFP transduction in multiple organs). AAV packaging size is normally constrained within 4.7 Kbp.