The 2009 pandemic H1N1 influenza virus encodes an NS1 protein with 11 amino acids (aa) truncation at the C-terminus.The C-terminal tail of influenza virus NS1 protein constitutes a nucleolar localization signal (NoLS) and is the binding domain of the cellular pre-mRNA processing protein,poly(A)-binding protein Ⅱ (PABⅡ).Here,our studies showed that the C-terminaltruncated NS1 of the 2009 pandemic virus was inefficient at blocking host gene expression,extension of the truncated NS1 to its full length increased the inhibition of host gene expression.