Recoding cassettes may be composed of a variety of diverse sequence elements. For example, primary nucleotide sequences may promote re-arrangements of tRNA molecules relative to their codons in mRNA inside the ribosome or affect recognition of tRNAs or release factors in the ribosomal A-site. On the other hand, many recoding signals act in the form of RNA secondary structures, such as simple stem-loops, or more complex pseudoknots, kissing stem-loops and other structures that involve interactions between considerably distant RNA regions (19, (25) (26) (27) (28) . Trans-acting RNA signals affecting ribosomal decoding through complementary interactions with ribosomal RNA (29-32), or through the nascent peptide acting within the ribosome exit tunnel (6, 33, 34) , are also known. Some recoding events-such as selenocysteine insertion-require the presence of additional specialized machinery such as selenocysteine tRNAs, selenocysteine-specific translation factors and several other components of the selenocysteine biosynthesis and insertion pathway (20, (35) (36) (37) . Recent reviews on stimulatory signals involved in the modulation of recoding events and molecular mechanisms of recoding provide further details (7, 25, 27, 38, 39) .