Interferon beta (IFNβ) modulates the expression of numerous cellular microRNAs (miRNAs), some of which target the hepatitis C virus (HCV) genome. Eight IFNβ-induced miRNAs show sequence complementarity with HCV RNA, and their introduction mimics the antiviral effects of IFNβ on HCV replication. Conversely, neutralizing these miRNAs with anti-miRNAs reduces IFNβ's antiviral effects. IFNβ also reduces the expression of miR-122, a liver-specific miRNA essential for HCV replication. These findings suggest that mammalian cells use cellular miRNAs, regulated by the interferon system, to combat viral infections. MiRNAs are small non-coding RNA molecules that regulate gene expression through RNA interference (RNAi). They are processed from primary transcripts by enzymes such as Drosha and Dicer, and then loaded into the RNA-induced silencing complex (RISC), where they guide RISC to complementary mRNAs. The seed region of miRNAs, particularly the 6–8 base pair sequence at the 5' end, determines their specificity for target mRNAs. RNAi was first recognized as an antiviral defense in plants, but its role in mammals has been debated. While the interferon system is considered the primary antiviral defense in mammals, this study shows that IFNβ can induce miRNAs that target viral transcripts, using RNAi as part of their antiviral strategy. Using microarray analysis, the study identified several miRNAs whose expression is regulated by IFNβ or IFNγ. Sequence complementarity analysis revealed that eight IFNβ-induced miRNAs have nearly perfect complementarity with HCV RNA. These miRNAs were tested in Huh7 cells, and their introduction significantly reduced HCV replication. Anti-miR-122 also reduced HCV replication, indicating that miR-122 is essential for HCV replication. To confirm that these miRNAs target the HCV genome, the study used a chimaeric virus with mutated target sites. miR-196 and miR-448 were effective against the wild-type virus but not the chimaeric one, indicating direct targeting of the HCV genome. The study also showed that modulating the expression of these miRNAs is important for the antiviral effects of IFNβ, although other factors may also contribute. In summary, this study demonstrates that IFNα/β upregulates several cellular miRNAs that inhibit HCV replication and infection. The downregulation of miR-122 further contributes to the antiviral effects of IFNβ. These findings provide new insights into the host defense mechanisms in mammalian cells and expand the antiviral arsenal of interferons. The study also highlights the potential of miRNAs asInterferon beta (IFNβ) modulates the expression of numerous cellular microRNAs (miRNAs), some of which target the hepatitis C virus (HCV) genome. Eight IFNβ-induced miRNAs show sequence complementarity with HCV RNA, and their introduction mimics the antiviral effects of IFNβ on HCV replication. Conversely, neutralizing these miRNAs with anti-miRNAs reduces IFNβ's antiviral effects. IFNβ also reduces the expression of miR-122, a liver-specific miRNA essential for HCV replication. These findings suggest that mammalian cells use cellular miRNAs, regulated by the interferon system, to combat viral infections. MiRNAs are small non-coding RNA molecules that regulate gene expression through RNA interference (RNAi). They are processed from primary transcripts by enzymes such as Drosha and Dicer, and then loaded into the RNA-induced silencing complex (RISC), where they guide RISC to complementary mRNAs. The seed region of miRNAs, particularly the 6–8 base pair sequence at the 5' end, determines their specificity for target mRNAs. RNAi was first recognized as an antiviral defense in plants, but its role in mammals has been debated. While the interferon system is considered the primary antiviral defense in mammals, this study shows that IFNβ can induce miRNAs that target viral transcripts, using RNAi as part of their antiviral strategy. Using microarray analysis, the study identified several miRNAs whose expression is regulated by IFNβ or IFNγ. Sequence complementarity analysis revealed that eight IFNβ-induced miRNAs have nearly perfect complementarity with HCV RNA. These miRNAs were tested in Huh7 cells, and their introduction significantly reduced HCV replication. Anti-miR-122 also reduced HCV replication, indicating that miR-122 is essential for HCV replication. To confirm that these miRNAs target the HCV genome, the study used a chimaeric virus with mutated target sites. miR-196 and miR-448 were effective against the wild-type virus but not the chimaeric one, indicating direct targeting of the HCV genome. The study also showed that modulating the expression of these miRNAs is important for the antiviral effects of IFNβ, although other factors may also contribute. In summary, this study demonstrates that IFNα/β upregulates several cellular miRNAs that inhibit HCV replication and infection. The downregulation of miR-122 further contributes to the antiviral effects of IFNβ. These findings provide new insights into the host defense mechanisms in mammalian cells and expand the antiviral arsenal of interferons. The study also highlights the potential of miRNAs as