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xp s nging by aminoacyl-tRNA synthetases (aaRSs) and during mRNA decoding by the ribosome. Aminoacylation errors are mainly caused either by failure of the aaRSs to differentiate between amino acids with similar chemical properties or by the incorrect recognition of tRNAs. Such errors are minimized by aaRSs editing mechanisms, which discard incorrectly bound amino acids, and by highly specific tRNA–aaRS interaction networks [1]. At the ribosome level, mRNA decoding is affected by four main types of errors: first, missense errors cause The evolutionary relevance of genetic code ambiguity is unequivocally demonstrated by the natural reassignments of UGA andUAG stop codons to selenocysteine (Sec) and pyrrolysine (Pyl), respectively, and by the widespread tRNA-dependent synthesis of asparagine (Asn), gluta-mine (Gln), and cysteine (Cys) [11,12,13], as discussed below. Selenocysteine (Sec) replaces cysteine (Cys) in the active site of a group of redox proteins generically known as www.sciencedirect.com Current Opinion in Microbiology 2009, 12:1–7Genetic code ambiguity: an une innovation and phenotypic dive