Recoding can benefit gene expression in a number of ways. It can regulate gene expression by being part of a sensor for particular cellular conditions. Prominent examples include ribosomal frameshifting in bacterial release factor 2 (RF2) and eukaryotic antizyme mRNAs. In both instances, ribosomal frameshifting is required for the production of the corresponding active full-length protein products. In the RF2 mRNA, the efficiency of frameshifting is negatively regulated by the cellular concentration of its product, RF2, providing an autoregulatory circuit for its biosynthesis (8) (9) (10) . In the antizyme mRNA, the efficiency of frameshifting is modulated by cellular levels of polyamines, whose concentration in turn is controlled by antizyme (11, 12) . Thus, this mechanism ensures the maintenance of antizyme production at the levels required to support physiologically appropriate concentrations of polyamines. Recoding can also be used for the diversification of protein products encoded by a single gene. An illustrative example is in bacterial dnaX mRNA, where frameshifting allows synthesis of two different protein subunits-sharing the same N-terminal part-from a single open reading frame (ORF) in its mRNA (13) (14) (15) . A presumed constant ratio of frameshifting in dnaX ensures a fixed stoichiometric balance between these two subunits (16) . This balance, then, is independent of the absolute levels of dnaX transcription and translational initiation on its mRNA. Similarly, in many viruses recoding is responsible for setting a ratio between protein products (such as those encoded by gag-pro-pol genes in retroviruses) produced from a single mRNA (17) . Recoding also provides RNA viruses with a mechanism for the translation of downstream ORFs on polycistronic RNAs [other mechanisms include leaky scanning, shunting, reinitiation, IRESs and the production of subgenomic RNAs (18) ] and may also be involved in global regulation mechanisms, such as mediating the switch between translation and replication on the same genomic RNA (19) . Finally, recoding provides a way for the incorporation of non-standard amino acids-e.g. amino acids that share their codons with termination signals (the most prominent example of which is selenocysteine, encoded by UGA) (20) (21) (22) . For further information on the diverse variety of recoding functions, see recent reviews (1, 3, 7, 23, 24) .