Th e key limitation of nonviral vector approaches has been their lack of effi ciency in mediating gene transfer and transgene expression in the airway epithelium. Viral vectors are immunogenic, due to the protein coat of the viral vector, and the immune response is related to both vector dose and number of administrations. Th e potential to limit administration to a single dose in ALI/ARDS may reduce this risk. However, the development of an infl amma tory response resulting in death following administration of a fi rst-generation adenoviral vector highlights the risks involved [48] . Additional limitations of viral vectors include transgene size, which is limited by the size of the capsid that encloses the viral genes.