The programmed À1 frameshifting is triggered by several elements in the mRNA. The slippery sequence, showing the X-XXY-YYZ motif, in which X can be any base, Y is usually A or U, and Z is any base but G, has the function of favouring the tRNA misalignment and it is the site where the shift takes place (3, 18) . Frameshifting could be further stimulated by other elements flanking the slippery sequence: a codon for a low-abundance tRNA, a stop codon, a Shine-Dalgarno sequence and an mRNA secondary structure. It has been *To whom correspondence should be addressed. Tel: +39 081 6132271; Fax: +39 081 6132277; Email: m.moracci@ibp.cnr.it Ó 2006 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.