A selection of above hairpins was cloned into a dualluciferase reporter plasmid and their frameshifting efficiency assayed in mammalian cells ( Figure 2C) . Although the absolute level of frameshifting was lower than in vitro, the trend was similar and showed maximal frameshifting of $8% around 12-15 bp. The pseudoknot in these assays was 1.6 times more efficient than the 12 and 15 bp hairpins, close to the in vitro ratio of 1.4 (see above). Thus, the hairpin derivative can effectively substitute for the SRV-1 pseudoknot in À1 ribosomal frameshifting.