A21. This nucleotide forms a base triple with the C2-G18 base pair. Mutation of A21 to G did not affect frameshifting, but replacement with C or U decreased the relative frameshifting activity to 60 and 63%, respectively (Table 1) . Although a guanine cannot substitute for the triple interaction with the C2-G18 base pair, its stacking properties may compensate for this. The two pyrimidines may be too small to bridge the distance here to form an interaction with C2-G18.