How could the nature of the 2b protein affect the selection of recombinant RNAs? It is conceivable that the selection of recombinant RNAs is a manifestation of the RNA silencing suppression-related functions of the 2b proteins, since some of these also vary between strains of CMV (57, 58) . However, based on the spatial separation of the 2b protein from the replication machinery within the cell (49) (50) (51) , it is difficult to envision how different 2b proteins provide the selection observed through their RNA silencing suppressor functions. In the case of the tombusvirus cucumber necrosis virus, inhibition of expression of the 20-kDa protein now known to be the RNA silencing suppressor of this virus, led to the rapid appearance of defective-interfering RNAs (59) . Thus, in that, the absence of the silencing suppressor led to enhanced generation or selection of recombinant RNAs, while in the case of CMV, the source of the 2b protein determined the nature of the recombinant RNAs selected and pseudorecombinant viruses could not be selected in its absence (21) . Moreover, in the case of influenza virus, where the NS1 gene encodes an RNA silencing suppressor (60) , mutation in a different gene (NS2) led to the rapid appearance of defective interfering particles (61) . At the time it was thought that the influenza NS2 protein might be involved in replication. In fact, the NS2 protein is involved in selecting ribonucleoprotein complexes for transport from the nucleus to the cytoplasm and is now called the nuclear export protein (62) .