Figure 2.

Operation rules for the expansion of signaling networks (a) A inhibits B, so two complementary nodes and are introduced as the logical negations of A and B, respectively. The inhibitory edge from A to B is replaced with an activating edge from A to and an activating edge from to B. (b) The interactions from A and B to C are conditional on each other, so a composite node AB is introduced. The activation of AB depends on the presence of A and B. The activation of C depends on AB. (c) The activation of C requires both the presence of A and the absence of B. The two operations illustrated in (a) and (b) are executed in combination to expand a signaling network.