FIGURE 1 

Definition of a module: two biochemical systems, M1 and M2, have defined input and output characteristics. M1 and M2 can be considered modules when we are able to predict the behavior of the composite network M2(M1) from the input/output characteristics of the individual systems. Interestingly, random biochemical networks connected together do not behave this way and therefore cannot be considered modules. The ability to predict the behavior of M2(M1) requires minimal retroactivity between M1 and M2 and the networks have to be either specifically engineered or evolved through a genetic algorithm for this to be the case.