All living things use two pairs of DNA nucleotides, or bases, to write their DNA. Now, researchers have created a microbe capable of adding a third pair. So in addition to the usual A-T and C-G base pairs, there is now X and Y. Not only do the designer microbes take up presynthesized X and Y nucleotides introduced into the surrounding culture medium, but they also use them to copy genes containing X-Y pairs. The advance opens the way for creating a wide variety of proteins with amino acids other than the standard 20 used in biology, which could lead to new protein-based materials and pharmaceuticals.