RNA interference(RNAi)is a process of eukaryotic posttranscriptional gene silencing that functions in antiviral immunity in plants,nematodes,and insects.However,recent studies provided strong supports that RNAi also plays a role in antiviral mechanism in mammalian cells.To combat RNAi-mediated antiviral responses,many viruses encode viral suppressors of RNA silencing(VSR)to facilitate their replication.VSRs have been widely studied for plant and insect viruses,but only a few have been defined for mammalian viruses currently.We identified a novel VSR from coronaviruses,a group of medically important mammalian viruses including Severe acute respiratory syndrome coronavirus(SARS-CoV),and showed that the nucleocapsid protein(N protein)of coronaviruses suppresses RNAi triggered by either short hairpin RNAs or small interfering RNAs in mammalian cells.Mouse hepatitis virus(MHV)is closely related to SARS-CoV in the familyCoronaviridaeand was used as a coronavirus replication model.The replication of MHV increased when the N proteins were expressedin trans,while knockdown of Dicerl or Ago2 transcripts facilitated the MHV replication in mammalian cells.These results support the hypothesis that RNAi is a part of the antiviral immunity responses in mammalian cells.