AbstractIFN‐α combined with ribavirin is used for the treatment of chronic hepatitis C. However, HCV has mechanisms to resist the antiviral actions of IFN‐α. In order to study the molecular mechanisms of this resistance, the effect of HCV gene expression on IFN‐induced nuclear import of STAT transcription factors and the expression of antiviral MxA protein were studied. In transiently transfected hepatoma cells, HCV core and NS5A proteins clearly inhibited the nuclear import of STAT1 and MxA protein expression (core only), whereas other viral proteins had only a marginal effect. To confirm these observations, human osteosarcoma‐derived cell lines, which inducibly express HCV core protein, the entire structural region (core‐E1‐E2‐p7), the NS3‐4A complex, NS4B, NS5A, or NS5B proteins were also used. IFN‐induced nuclear accumulation of STAT1 was almost completely and STAT2 was partially blocked in cell lines expressing high levels of HCV core protein. Subsequently, in these cells, IFN‐α‐induced MxB protein expression was decreased. Tumor necrosis factor‐α (TNF‐α)‐induced nuclear import of NF‐κB was only weakly or not at all inhibited, suggesting that the nuclear import machinery in general was not impaired. The results demonstrate a novel mechanism by which HCV gene expression may interfere with IFN‐mediated host defence systems. J. Med. Virol. 73:536–547, 2004. © 2004 Wiley‐Liss, Inc.