The detection of virus infection by these receptors leads to the induction of interferons and their downstream IFNinducible anti-viral genes through distinct signaling pathways. Type I IFN is an important regulator of viral infections in the innate immune system. Another type of IFN, IFN-lambda, affects the prognosis of HCV infection, and its response to antiviral therapy. Mutations impairing the function of the RIG-I gene and the induction of IFN were essential in establishing HCV infectivity in human HuH7.5 cells. Similarly, the HCV-NS3/4a protease is known to cleave IPS-1 adaptor molecule, inducing further downstream blocking of the IFN-inducing signaling pathway. These data clearly demonstrate that the host RIG-I pathway is crucial for suppressing HCV proliferation in human hepatocytes. Using a similar strategy, we investigated whether suppressing the antiviral host innate immune DAF-FM system conferred any advantage on HCV proliferation in mouse hepatocytes. We examined the possibility of HCV replication in mice lacking the expression of key factors that modulate the type I IFN-inducing pathways. Only gene silencing of the IFN receptor or IPS-1 was sufficient to establish spontaneous HCV replication in mouse hepatocytes. To establish a cell line permissive for HCV replication, which is required for further in vitro studies of the HCV life cycle in mouse hepatocytes, we immortalized IFNAR- and IPS-1-knockout mice hepatocytes with SV40 T CGP-37157 antigen. Upon expression of the human CD81 gene, these newly established cell lines were able to support HCV infection for the first time in mouse hepatocytes. Viral factors required for HCV replication in mouse hepatocytes were also analyzed. As a first step in establishing HCV infection in mice, we tested the susceptibility of mouse hepatocytes to persistent expression of HCV proteins after RNA transfection. In vitro transcribed chimeric J6JFH1 RNA, in which the HCV structural and non-structural regions were from J6 and JFH1 isolates respectively, was transfected into hepatocytes from wild-type mice. We used a highly sensitive polyclonal antibody derived from HCV-patient serum for the detection of HCV proteins. No HCV proteins were detected five days after transfection, suggesting that wildtype mouse hepatocytes were unable to maintain HCV replication. We then tried to find and block the pathway used by mouse hepatocytes for the detection of viral-RNA and the induction of IFN response.