It turns out that your biology teacher (and a certain 1997 sci-fi flick) got something wrong -- DNA isn't necessarily limited to four letters. Scripps Research Institute scientists tell Wired that they've created living cells which include two artificial letters (that is, nucleotides) in their genetic code in addition to the naturally occurring A, C, G and T. The researchers' primary obstacle was making sure these nucleotides cooperated with the enzymes that copy and transcribe DNA; after that, it was just a matter of getting some E. coli bacteria to accept and propagate the newly augmented sequences.It could be a long while before we see any practical uses of the breakthrough. It's expensive to create the necessary precursor molecules, and it's not yet possible to get cells making those precursors all on their own. Should the processes become cheaper and easier, though, extending DNA could be helpful for testing the effects of medicine on certain gene combinations. It could also be used to deliver gene therapy; you could have tailor-made DNA that combats specific problems. Whatever comes next, it's now obvious that nature has a pretty limited vocabulary.