Expanding the genetic lexicon: incorporating non-standard amino acids into proteins by ribosome-based synthesis.

Only 20 amino acids are normally incorporated into proteins synthesized in living cells, and this has limited the structural range of proteins that can be prepared. New methods that allow the incorporation of amino acids that are not normally encoded by natural genes are being developed: these include reassigning functions within the existing genetic code, and expanding the genetic code by constructing additional, non-natural codons. Used in conjunction with recent major advances in understanding protein structure-function relationships, these approaches should extend the range of de novo protein designs that are possible.