wabrandsma writes "New Scientist reports that 'A bacterium has had its genome recoded so that the standard language of life no longer applies. Instead, one of its words has been freed up to impart a different meaning, allowing the addition of genetic elements that don't exist in nature. ... The four letters of the genetic code are usually read by a cell's protein-production machinery, the ribosome, in sets of three letters called codons. Each codon "word" provides instructions about which amino acid to add next to a growing peptide chain. Although there are 64 ways of combining four letters, only 61 codons are used to encode the 20 amino acids found in nature. ... The three combinations left over, UAG, UAA and UGA, act like a full stop or period – telling the ribosome to terminate the process at that point. ... A team of synthetic biologists led by Farren Isaacs at Yale University have now fundamentally rewritten these rules (abstract). They took Escherichia coli cells and replaced all of their UAG stop codons with UAAs. They also deleted the instructions for making the release factor that usually binds to UAG, rendering UAG meaningless. Next they set about assigning UAG a new meaning, by designing molecules called tRNAs and accompanying enzymes that would attach an unnatural amino acid – fed to the cell – whenever they spotted this codon."