In the case of C1C2W 2B T3 Á163(G) , switchover occurred following nucleotide 2064 in stem-loop G, but did not immediately proceed to the (À) CMV RNA 2 as the receptor RNA. Rather, the polymerase jumped back to the subgenomic promoter stem-loop (L2) and copied this through the A residue in the gap between stem-loops L2 and K2 ( Figure 7A) , before pausing and jumping to the same subgenomic promoter sequence on (À) CMV RNA 2. The small difference in ÁG between the 6 bp stem-loop K2 in (À) TAV RNA 3 (ÁG = À2.7 kcal/mol) and a weaker 3 bp stem-loop (K 0 ) structure adjacent to the subgenomic promoter in (À) CMV RNA 2 (ÁG = À2.1 kcal/mol) may be sufficient to prevent the polymerase from pausing at the base of stem-loop K 0 in (À) CMV RNA 2. This explains the duplication of the 19 nt seen in Figure 5D , following the A residue (from copying the first nucleotide, U-1902 of the RNA 5/3B promoter). In the case of C1C2 W2B T3, the same recombinant progeny RNA 3 is seen, but it must have been generated from a different starting point, since the second repeat is present in the template but missing in the recombinant progeny RNA 3. Here, template switching occurred either immediately after nucleotide 2084 (the A residue in the gap between stem-loops L1 and K1), again leading to a partially duplicated subgenomic promoter with the 19 nt repeat, or it occurred at nucleotide 2064, in the lower region of stem-loop L1 ( Figure 7B ), and then proceeded as described above for C1C2 W2B T3 Á163(G) .