Here, we focus on the frameshifting signal found in several viruses (1) , including infectious bronchitis virus (IBV) and SARS-CoV. The signal leads to programmed ribosomal À1 frameshift, whereby multiple proteins are produced from a single polycistronic messenger RNA (mRNA) (4, 5) . The frameshift efficiency, i.e. the fraction of ribosomes, which change reading frame, is important to ensure a correct stoichiometric relationship between the different products of translation. It has been shown that altered frameshift efficiency has detrimental effects on the proliferation of HIV-I and the yeast L-A viruses (6, 7) . In order to induce À1 frameshift, these viruses rely on three physical features on the mRNA: a heptanucleotide sequence, a spacer and a downstream structure (8) . The heptanucleotide sequence, called the slippery sequence, is where the À1 frameshift occurs and typically has the following sequence: X XXY YYZ, where X, Y and Z denote nucleotide species and spaces indicate initial reading frame. The spacer is a stretch of 6-9 nt positioning the ribosome correctly at the slippery site when encountering the downstream structure. The downstream structure is most often found to be a pseudoknot. The pseudoknot structure probably functions as a physical barrier deforming upon approach of the translating ribosome (9) , thereby assisting the frameshifting process; however, geometry and surface charge of the structure may also play a role for the frameshifting (10) .