Regarding Motif 1, it is clear that in all studies so far, mRNA mutations that would destabilise an interaction with 18S rRNA reduce or abolish reinitiation and changes not predicted to affect pairing having a lesser effect or none at all. Recently, the reciprocal experiment was performed, where mutations were introduced into the relevant region of (yeast) 18S rRNA. Their effect on termination-reinitiation was found to be highly consistent with a role for mRNA-18S rRNA pairing [18] . These experiments confirm a role in tethering through rRNA, although do not rule out the contribution of other factors, for example, binding of eIF3 [13] . A comparative alignment of the MNV signal with other known or suspected termination-reinitiation signals (Figure 8 ) reveals that Motif 1 is always present and that the stop and start codons of the termination-reinitiation site are in close proximity to each other. What does vary is the spacing between the two elements, from only 26 nt in the case of BM2 to 29 nt in MNV, 53 nt in FCV, 61 nt in RHDV and 62 nt (the longest) in the Lagovirus European brown hare syndrome virus. It is not clear whether the ''additional'' sequences present in viruses with longer TURBS have a role in termination-reinitiation. Deletion analysis of the FCV and RHDV TURBS has revealed some dispensible sequences -there may be some flexibility in the spacing of Motif 1 that allows other biological information to be accommodated into the TURBS without affecting function in stop-start. However, there is little sequence conservation between the signals of viruses of different genera, arguing against the presence of other primary sequence motifs. Another stretch of bases of functional consequence has been identified in FCV and RHDV, namely TURBS Motif 2, which is located closer to the stop-start window than Motif 1 and is speculated to help position ribosomes correctly at the reinitiation codon [12, 14] . Recent work has shown that the functional requirement for Motif 2 is in its participation in a basepaired region that forms between this motif and a stretch of bases immediately upstream of Motif 1 [18] ; see Figure 8 . This basepairing has previously been noted from structure predictions of the signal of FCV [13] and direct RNA secondary structure probing of BM2 stem 2 [19] and the MNV stem 2 (see Figure 4) . Based on the observations of Luttermann and Meyers [18] , the formation of this stem is likely to be important to termination-reinitiation in the BM2 and MNV systems. Indeed, it is noticeable that in the deletion analysis of the MNV signal, and that of BM2 [19] , those deletions that would affect formation of stem 2 showed reduced activity in termination-reinitiation (Figure 1b, Figure 4) .