A large number of differentially expressed proteins were identified from the nuclear cell fraction at 12 and 18 h postinfection (Fig. 1C ). In the nuclear fraction 335 and 376 proteins were overexpressed, and 344 and 368 proteins underexpressed at 12 and 18 h post-infection, respectively. The overexpressed proteins contain several mitochondrial, ER, Golgi and cytosolic proteins. Functionally the overexpressed proteins form a heterogeneous group of proteins including several metabolism, gene expression, calcium ion binding, transport and signaling related proteins ( Fig. 2A) . Here, proteins related with gene expression are mainly nuclear or ribosomal proteins whereas proteins related with metabolism, transport, signaling and calcium ion binding originate from several different subcellular compartments. Proteins that are underexpressed in the nuclear fraction at 12 and 18 h postinfection are mainly known nuclear proteins. Almost 200 of them are involved in gene expression, especially in nucleotide metabolism and mRNA processing (Fig. 2B) . Host mRNA splicing machinery has been suggested to be important for influenza virus gene expression [18] , and interestingly, 40 proteins related with nuclear mRNA splicing were underexpressed in our data. Finally, our proteomics data showed that the amount of several nuclear histones increased clearly in the cytoplasmic fraction at 12 and