Genetic logic circuits are combinations of genes and regulatory sites that combine to form regulatory modules with close analogy to familiar modules from circuit design. A very simple example is a pair of genes, A and B, each of which encodes a transcription factor that can prevent expression of the other. This construct, a bistable switch memory module, is stable with A on and B off, or with B on and A off, but not in intermediate states.

Several classes of modules have been described in natural systems, or designed in synthetic biology applications. These include AND, OR, and NOR logic gates and memory modules, the basic elements needed to build more complex regulatory circuits. For example, Lou, et al. (PMID:20212522) describe a push-on push-off switch; the same signal toggles the system on if it is off, or off if it is on.

Genetic memory modules provide an obvious mechanism for epigenetic inheritance. However, the dominant mechanism for epigenetic inheritance in eukaryotic cells may be heritable patterns of DNA methylation rather than genetic logic circuits.