Published today in Nature, researchers report engineering an organism capable of replicating DNA with an unnatural base pair (X-Y) in addition to the two normal base pairs found in DNA (A-T and C-G). Here's what the unnatural base pair looks like (in comparison to the C-G base pair): Here's the abstract for the article: Malyshev et al. A semi-synthetic organism with an expanded genetic alphabet. Nature. Published online 07 May 2014. http://dx.doi.org/10.1038/nature13314 [Broken]. It'll be interesting to see what (if any) applications come from this work. It seems like there's still some work to be done in order to make it a little easier for these organisms to naturally use the new bases (for example, maybe getting the organisms to synthesize them by themselves).