Programmed À1 ribosomal frameshift signals are typically divided into three components. From 5 0 to 3 0 these are (i) a 'slippery site' in the form N NNW WWH, where N must be a stretch of any three identical nucleotides, where W is either three A or U residues, and H is A, C or U (spacing indicates the unshifted zero frame), (ii) a spacer region and (iii) an mRNA structural element, most often a pseudoknot. The general model posits that upon encountering the mRNA pseudoknot, an elongating ribosome is forced to pause such that the anticodons of its A-and P-site tRNAs are base-paired with the zero-frame codons of the slippery site. The nature of the tRNA-mRNA interactions is such that a relative slip of À1 nucleotide still allows base-pairing in the non-wobble positions. The slippage occurs during the ribosomal pause, and it has been shown that changes affecting ribosome pause times affect frameshift efficiencies [reviewed in (7) ]. An important observation is that even though mRNA pseudoknots and energetically equivalent stem-loop structures appear to promote ribosome pausing with equal effectiveness, mRNA pseudoknots are more efficient at promoting À1 PRF (8). Our '9 Å ' model (6) provided a refinement of the original 'simultaneous slippage' (9, 10) model of frameshifting by suggesting that rather than the entire ribosome having to slip one base in the 5 0 direction, slippage could be accomplished by moving the small section of mRNA in the downstream tunnel by one base in the 3 0 direction. We have proposed that this is accomplished by the bulky and difficult to unwind mRNA pseudoknot structures becoming wedged in the downstream entrance tunnel of the ribosome, preventing the downstream region of the mRNA from being pulled into the ribosome by the equivalent of one base during the accommodation step of elongation. This blockage would introduce tension into the spacer region, which could be resolved by unpairing the mRNA from the tRNAs, allowing the mRNA to slip 1 nt backwards, resulting in a net shift of reading frame by À1 base.