Truth tables of transcriptional logic gates.

†The act and inh gates have only one input; they depend on only.
Logic gates are devices that perform elementary binary computations, mapping multiple input signals to one output signal. Here we consider transcriptional logic gates. Transcription systems are analogue systems and therefore never truly binary, but it is often useful to think of them as continuous approximations of logic gates. Here, we consider logic gates with one or two inputs (the concentrations and of two transcription factors), and one output (the steady-state expression level of the regulated gene: ). The table specifies, for the six logic gates used in our study, the value of the appropriate output (“low” or “high”) for all input concentrations and (“low” or “high”). All gates are defined for input concentrations in the domain only; concentrations above 500nM are considered high and those below 500nM are low. The acronyms of the gates summarize their function; for instance, the output of an AND gate should be high only when both and are high. Note that that by definition the ACT (activate) and IN (inhibit) gates have only one input, ; indeed, their output does not depend on .