Life is picky.
Organisms incorporate only 20 amino acids into proteins, while chemists can make hundreds of such building blocks. Researchers have long wanted to use these amino acids to create proteins that can carry out new functions. But getting living organisms to take those steps has proven impossible, until now. If the editor finds a mistake, it clips off the incorrect amino acid, giving the tRNA a new chance to load the right one. So Schimmel and his colleagues simply mutated the editing portion of one aaRS, a change that allowed it to load unnatural amino acids onto tRNAs, which were then readily incorporated in proteins.