[MicroRNAs and viral infections]

RNA silencing plays an important role in development through the action of micro (mi) RNAs that fine tune the expression of a large portion of the genome. But, in plants and insects, it is also a very important player in innate immune responses, especially in antiviral defense. It is now well established that the RNA silencing machinery targets plant as well as insect viruses. As the genetic basis underlying this defense mechanism in these organisms starts being elucidated, much less is known about the possible antiviral role ofRNAsilencing in mammals. No small interferingRNAs (siRNAs) of viral origin can be detected in human cells infected withRNAviruses. On the contrary, viral smallRNAs can be found in cells infected by the DNA virus Epstein-Barr, but they rather resemble miRNAs. This primary observation has been extended to other members of the herpesvirus family as well as other DNA viruses such as the polyomavirus SV40. These findings indicate that, rather than being targeted by RNA silencing, human DNA viruses seem to have evolved their own miRNAs to modulate the expression of their own genes or of their host's.