RNA viruses should deal with the lack of the quality control of mRNA capping in the cytoplasm

 Cellular mRNAs are readily capped after initiation of transcription, whereas it likely happens that the capping machinery sometimes fails to cap them. Cells have preparations for such situation by sensing and removing these incompletely capped mRNAs. This capping quality control activity takes place in the nucleus, because, needless to say, cellular gene expression is conducted in the nucleus. Interestingly, many RNA viruses transcribe mRNAs in the cytoplasm by using their own RNA dependent RNA polymerases. Therefore, if incomplete capped mRNAs are produced in the cytoplasm, it means that there is not a cap quality control mechanism for RNA viruses. It is considered that such incomplete mRNA is recognized as an abnormal mRNA by cellular mechanisms, and subsequently, it induces the activation of innate immunity. Viruses must deal with such cellular antiviral attack.