To examine how soon after infection this 19-nt repeat was generated and whether there was an intermediate generated before this 19-nt repeat appeared, a timecourse experiment was done, in which two viruses, C1C2 T2B T3 Á163(G) and C1C2 W2B T3 Á163(G) , were inoculated onto N. clevelandii. Nicotiana clevelandii was chosen as the host because the symptoms are more readily seen on this host than on N. glutinosa (unpublished data) and C1C2 T2B T3 Á163(G) and C1C2 W2B T3 Á163(G) were chosen because both viruses showed more rapid appearance of recombinant RNA 3 (see above). Moreover, the recombinant progeny virus of C1C2 W2B T3 Á163(G) contained the 19-nt repeat, which could be used as a marker to determine whether there was an intermediate before this final product. After inoculation of each virus to 200 plants, leaves were detached from groups of five inoculated plants at 1-day intervals for up to 40 days. Northern blot hybridization of the total RNAs extracted from the detached leaves showed that recombinant RNA 3s derived from any of the above viruses did not accumulate to a detectable level before day 35; the earliest time when a recombinant RNA 3 could be detected by northern blot hybridization was at day 35 in the C1C2 T2B T3 Á163(G)infected plants (Supplementary Figure 1B) . The recombinant RNA 3 derived from C1C2 W2B T3 Á163(G) was not detected during the whole time-course of the experiment (data not shown). However, by using RT-PCR with the primer pair C3 0 /T3-1187, the recombinant RNA 3 derived from C1C2 T2B T3 Á163(G) was detected as early as day 22 (Figure 6 upper panel) , while the RNA 3 recombinant derived from C1C2 W2B T3 Á163(G) could be detected at day 35 (data not shown). These results also confirmed that the TAV 2b protein promoted the recombination/selection faster than the WAII-CMV 2b protein.