The notion that mechanical stability of mRNA structural element is crucial for -1 PRF, has triggered mechanical unfolding experiments of individual mRNA pseudoknot, mutants as well as some of the constituent hairpins using optical tweezers hairpins (19) (20) (21) . Some of these experiments suggest that frameshift efficiencies correlate with unfolding forces rather than the free-energy difference between the folded and unfolded state (21) . This indicates that -1 PRF is kinetically controlled, as has been proposed previously (22) .