Figure 5. Nodes can become coupled via common downstream targets.
(A) Node 
has two parents: 
and 
, without a regulatory path between them. (B) Perturbation 
applied to 
. (C) In the case of competitive binding, increasing the concentration of free 
yields more of 
bound to the promoter of 
, forcing some of the molecules of 
to unbind, thus increasing the free concentration 
. Consequently, 
acts as if it were an activator of 
. (D) By contrast, in the case of cooperative binding, when the binding of 
must precede that of 
, pulses in 
yield pulses of the opposite sign in 
. Consequently, 
acts as if it were a repressor of 
. Simulation parameters are: 
, 
, 
, 
, 
, 
, and both 
and 
bind as tetramers.