Although the first miRNA was identified 18 years ago, it was only in 2001, with the development of technologies that allowed the efficient cDNA cloning and sequencing of small RNA species, that it became apparent that all multicellular eukaryotes encode numerous members of this class of small regulatory RNAs. Shortly after the identification of the first human miRNAs, the first virally encoded miRNAs were reported in the human herpesvirus Epstein–Barr virus (EBV). Although this initial discovery suggested that viruses in general might use miRNAs to down-regulate cellular factors that inhibit viral replication, the subsequent analysis of a wide range of RNA viruses failed to identify any viral miRNAs. However, virally encoded miRNAs are expressed by many members of the herpesvirus family of nuclear DNA viruses and are also found in a small number of other nuclear DNA viruses, particularly polymaviruses.
Although >250 viral microRNAs (miRNAs) are expressed by a range of nuclear DNA viruses, efforts to identify miRNAs expressed by RNA viruses have so far been in vain. In PNAS, Kincaid et al. report the identification of five miRNAs encoded by the delta retrovirus bovine leukemia virus (BLV) that are expressed in BLV-transformed B cells. It appears likely that these viral miRNAs play an important role in BLV pathogenesis: