ic messenger RNA. This structure plays several key biological functions such as protection of RNA from 5′-exoribonucleoases and initiation of the RNA translation into proteins. Moreover the cap is a marker of ‘self’ preventing detection by the cellular innate immunity mechanism. Thus viruses such as flaviviruses and coronaviruses code for RNA capping pathway mimicking that of eukaryotic cells. These viruses produce a subset of enzymatic activities including a RNA 5′-triphosphatase, a guanilyltransferase (GTase) and