Although SF224 is a slightly better shifter than SF206 (1.11-fold) with an absolute frameshift frequency of 24% it still cannot be considered a supershifter. Previous experiments with SRV-1 pseudoknot mutants yielded better shifters (31) . One of these, SF67, which differed from . Figure 2 . Putative alternative structure of the 'supershifter' pseudoknots. The boxed G-C base pairs and the circled G are also present in the wild-type SRV-1 pseudoknot. The position of the C24 deletion is indicated by the dot and arrowhead.