Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal. Functional analysis of the SRV-1 RNA frameshifting pseudoknot Simian retrovirus type-1 uses programmed ribosomal frameshifting to control expression of the Gag-Pol polyprotein from overlapping gag and pol open-reading frames. The frameshifting signal consists of a heptanucleotide slippery sequence and a downstream-located 12-base pair pseudoknot. The solution structure of this pseudoknot, previously solved by NMR [Michiels,P.J., Versleijen,A.A., Verlaan,P.W., Pleij,C.W., Hilbers,C.W. and Heus,H.A. (2001) Solution structure of the pseudoknot of SRV-1 RNA, involved in ribosomal frameshifting. J. Mol. Biol., 310, 1109–1123] has a classical H-type fold and forms an extended triple helix by interactions between loop 2 and the minor groove of stem 1 involving base–base and base–sugar contacts. A mutational analysis was performed to test the functional importance of the triple helix for −1 frameshifting in vitro. Changing bases in L2 or base pairs in S1 involved in a base triple resulted in a 2- to 5-fold decrease in frameshifting efficiency. Alterations in the length of L2 had adverse effects on frameshifting. The in vitro effects were well reproduced in vivo, although the effect of enlarging L2 was more dramatic in vivo. The putative role of refolding kinetics of frameshifter pseudoknots is discussed. Overall, the data emphasize the role of the triple helix in −1 frameshifting. Ribosomal frameshifting is a translational recoding mechanism that allows the synthesis of multiple proteins from a single mRNA. During this process a certain proportion of the ribosomes is forced to move one or two nucleotides backwards (-1 or À2 frameshift) or forwards (+1 or +2 frameshift) whereafter they continue translation in the new reading frame. As a result the stop codon of the first open-reading frame is bypassed and a fusion protein is synthesized [reviewed in (1, 2) ] Frameshifting is frequently used by RNA viruses, in particular by those with a single genome, and is thought to lead to precise ratios of viral proteins, which is crucial for successful infection (3, 4) . Frameshifting is occasionally used by the eukaryotic cell, e.g., to regulate expression of antizyme (5) or by the prokaryotic cell to regulate production of release factor RF2 (6) and synthesis of the gamma subunit of DNA polymerase (7) .