genescould be efficiently used to rescue viruses, we performed a virusrescue assay using one cloned gene from this study and sevenother plasmids from PR8. As described in Table 2, all of the clonedgenes worked well and could be used to rescue viruses. The viraltiter of the rescued viruses (1 + 7) without further passages couldreach 1.58 × 103to 1.58 × 107TCID50/ml. The H9N2 6PR8 couldbe easily rescued using the cloned HA and NA genes from theH9N2 field isolate and six internal genes from PR8, indicatingthe application of our strategy in influenza vaccine preparation.And the PR8 or WS1 viruses also could be efficiently rescuedusing all eight genes cloned in this study without further passage.These results proved that the approach developed here for cloninginfluenza genes for influenza reverse genetics studies was