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Base pairing — the chemical rule that determines how two strands of DNA bind together — provides the natural ingredients for molecular logic. The 'input' to a logic gate (an existing pair of DNA strands) is a single strand that binds to the pair and displaces one strand, which then forms the 'output'.

Scaling these DNA gates up to more complex circuits requires that they be easy to connect, and possess feedback and controls to eliminate errors early — the basic principles of digital design. Aided by computer optimization, Erik Winfree and co-workers1 from the California Institute of Technology in the USA have constructed circuits with up to 11 DNA-based logic gates that function in vitro. The advantage of these circuits is that they have built-in signal restoration — a feature that prevents output when the input is too low and ensures output when the input is correct.

The limiting factor, however, is speed — signal restoration costs a factor of five in time compared with simpler circuits. Yet, the fact that digital design principles have been successfully implemented at this scale may mean that in vivo 'data processing' is the next logical step.