Cystic fibrosis (CF) is the most common autosomal recessive disease. This lethal disease is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Currently, there is no cure. Progressive loss of lung function is the leading cause of morbidity and mortality in CF. Delivering a functional CFTR gene to airway cells may provide a permanent remedy for this relentless disorder. Adeno-associated virus (AAV) has been at the forefront for CF gene therapy. Several clinical trials have been performed with AAV vector. Despite promising results in safety and DNA level gene transfer, these trials have failed to yield enough CFTR protein in patients. The lack of a strong promoter is largely responsible for the low level protein expression. AAV is one of the smallest viruses. Its 5kb packaging capacity is barely enough to carry the full-length CFTR cDNA. There is no room for a potent promoter. Novel strategies are urgently needed to expand AAV packaging capacity. Several dual vector approaches, including trans-splicing and overlapping, hold promise to deliver a strong full- length CFTR cDNA expression cassette. Unfortunately, their transduction efficiency is too low to be useful. Furthermore, the success of these traditional dual vector approaches is highly dependent on the transgene sequence and they may not work for the CFTR gene. To overcome the inherent limitation in the traditional dual vector approaches, we have now developed the hybrid vector system, a noval second generation dual vector strategy. This is a generic approach independent of the transgene sequence. [unreadable] [unreadable] [unreadable]