The conserved 23-nt sequence at the beginning of the 3 0 NTR has been implicated in the recombination and nucleotides 1-20 of the 23-nt sequence also have been suggested to be an internal subgenomic promoter site (33, 38) . This sequence also was always near the crossover sites in the recombinant RNA 3s observed here, and the 19-nt imperfect repeat also comes from this sequence. Therefore, we examined the requirement for this sequence in infection. Three deletion mutants were constructed: C1 Á23 , C2 T2BÁ23 and T3 Á163(A)Á23 , where the conserved 23-nt sequence was deleted from each of C1, C2 T2B and T3 Á163(A) , respectively. Co-inoculation of each deletion mutant in the combinations C1 Á23 with C2 T2BÁ23 and T3 Á163(A)Á23 , C1 Á23 with C2 T2B and T3 Á163(A) , C1 with C2 T2BÁ23 and T3 Á163(A) , or C1 with C2 T2B and T3 Á163(A)Á23 , onto N. glutinosa and N. clevelandii plants, showed that none of these combinations was able to infect the plants (Table 1) . However, when either C1 Á23 was combined with C2 and C3, or T3 Á163(A)Á23 was combined with T1 and T2, the mixtures were infectious (Table 1) , indicating that the presence of this 23-nt sequence is essential for infection of the pseudorecombinant viruses, but not the parental viruses. Since the pseudorecombinant viruses containing the 23-nt sequence were viable without detectable recombination for some time (Figure 2) , these results indicate that the 23-nt sequence is not essential for replication of the pseudorecombinant virus, but may be essential for maintaining infection of the pseudorecombinant viruses through the plants, just as the 2b protein was required for this task, but not for the maintenance of infection of the parental viruses (19, 21) .