Gene therapy is the transfer of genetic material to a patient’s cells to achieve a therapeutic effect. Therapeutic DNA typically is delivered using a viral vector system, and adenoviruses have been used for this purpose for over 20 years (1–3). Within the past 10 years or so, lentiviruses have shown promise in clinical trials (1–3), and adenoassociated viruses (AAVs) have been used in the first approved gene therapies in the Western world (4). The number of gene therapy applications based…