Previous work on using organisms as circuitry has usually involved shoehorning parts of the digital world into a very analog environment. MIT has just found an approach that uses the subtlety of the natural world to its advantage: the circuits themselves are analog. By combining genes that produce similar molecules in response to different inputs, the school's scientists have created bacterial cells that perform basic math -- the exact quantity or ratio of a given molecule is the answer. The approach offers a much wider range of results than a binary circuit (10,000 versus 2), and it exploits the cell enzymes' inherent ratio awareness to do some of the hard work. MIT wants more variety in genetic ingredients before it can produce a truly universal system, but its work could lead to organic sensors that are much simpler and more precise than their digital peers.