The siRNAs targeting M or NP gene of influenza A virus were designed by siRNA target designer version 1.51 from Promega (http://www.promega.com/siRNADesigner/program/). The duplexes of designed and previously reported siRNAs were synthesized by Invitrogene (USA) (the sequences were shown in the supporting information Table S1 ). The siRNAs were reverse transfected to MDCK cells using Lipofectamine TM RNAiMAX (Invitrogene, USA) as described in company's instruction. After incubated the cells for 16,18 hrs, the cells were infected with the viruses and followed by detection of viral replication. 24 hours after infection, RNA were extracted from the cells and followed by real time RT-PCR to detect the relative quantities of replicated viral RNA.