From Scientific American:
Scientists at The Scripps Research Institute (TSRI) have engineered a bacterium whose genetic material includes an added pair of DNA “letters,” or bases, not found in nature. The cells of this unique bacterium can replicate the unnatural DNA bases more or less normally, for as long as the molecular building blocks are supplied. “Life on Earth in all its diversity is encoded by only two pairs of DNA bases, A-T and C-G, and what we've made is an organism that stably contains those two plus a third, unnatural pair of bases,” said TSRI Associate Professor Floyd E. Romesberg, who led the research team. “This shows that other solutions to storing information are possible and, of course, takes us closer to an expanded-DNA biology that will have many exciting applications — from new medicines to new kinds of nanotechnology.”
The report on the achievement appears May 7, 2014, in an advance online publication of the journal Nature. Romesberg and his laboratory have been working since the late 1990s to find pairs of molecules that could serve as new, functional DNA bases — and, in principle, could code for proteins and organisms that have never existed before. The task hasn't been a simple one. Any functional new pair of DNA bases would have to bind with an affinity comparable to that of the natural nucleoside base-pairs adenine-thymine and cytosine-guanine. Such new bases also would have to line up stably alongside the natural bases in a zipper-like stretch of DNA. They would be required to unzip and re-zip smoothly when worked on by natural polymerase enzymes during DNA replication and transcription into RNA. And somehow these nucleoside interlopers would have to avoid being attacked and removed by natural DNA-repair mechanisms.