Figure 1. Technique evolution of influenza virus reverse genetics. Helper virus depended influenza reverse genetics system; (2) 17-plasmid influenza reverse genetics system. Eight plasmid for synthesis of RNA segments, and 9 plasmids for expression of structural proteins; (3) 8-plasmid influenza reverse genetics system. Each plasmid has PolII and PolI promoter sequence flanked by influenza gene segments; (4) 3-plasmid influenza reverse genetics system. One plasmid carries eight PolI promoter-driven vRNA transcription units. The second plasmid encodes polymerases, and the third one expresses NP; (5) one-plasmid influenza reverse genetics system.