If we assume that ribosomes first sense the bottom of stem S1 we would expect base triples to be present at the bottom of the stem or in the 5 0 end of L2. This would be in line with the observation that the first three base pairs of a frameshift pseudoknot are usually G-C (30). However, most triples are found near the 3 0 end of L2 close to the junction with S2. Possibly, triples at the junction render the pseudoknot more resistant to forced unwinding by the ribosomal helicase, which would otherwise serve as a pivot.