Fig. 1.

A schematic diagram of a simple genetic network, consisting of four component modules, signified by ovals and circles (with the component, interacting genes not indicated). Each module's expression is evoked by an external signal that can be one derived external to the cell (environmental, cell matrix-derived, hormonal, etc.) or by an internal gene product from another genetic module. Three of the modules depicted here have outputs in the form of target gene sets including cytodifferentiation gene products (A, D, and C) whereas one module (B) is purely regulatory. (Adapted from ref. 18.)