The signals that are responsible for À1 frameshifting comprise two elements: a slippery sequence where the actual reading shift takes place, and a downstream located structural element which greatly stimulates the efficiency of frameshifting. Although the mechanism is still elusive, the present view is that the downstream structure forms a physical barrier that blocks EF-2 function and causes ribosomes to stall in their translocation step. This 'roadblock' puts tension on the mRNA-tRNA interaction. The tension can be relieved by the realigning of A-site and P-site tRNAs in the 5 0 -direction, whereafter EF-2 can do its work and the ribosome resumes translation in the À1 reading frame (3) .