Real-time RT-PCR was carried out as described previously [39] . Briefly, H1N1 or H5N1 virus infected MDCK, Mock MDCK, shNP-MDCK and shM2-MDCK were harvest at 1, 2, 4 and 24 hr after infection. Total RNA was extracted from the infected cell samples using RNeasy RNA isolation Kit (Qiagen, Germany) and reverse transcribed using Superscript II Reverse Transcriptase and Oligo dT primer (Invitrogene, USA), according to the manufacturer's protocol. Viral mRNA copies were measured by SYBR green M63000 Real-Time PCR System 