The protein expression plasmids encoding PB2, PB1, PA and NP (pcDNA-PB2, pcDNA-PB1, pcDNA-PA and pcDNA-NP) and transcription plasmids encoding eight IAV-WSN vRNA segments (pPOLI-PB2, pPOLI-PB1, pPOLI-PA, pPOLI-HA, pPOLI-NP, pPOLI-NA, pPOLI-M, and pPOLI-NS) were a kind gift of Dr. Ervin Fodor [37] .