Interaction diagrams and cis -regulatory input function. (A) Interaction diagrams describing the six gene regulatory networks used in the simulations. A circle represents a gene, a +(-)-signed arrow from gene A to gene B symbolises that the gene product of A activates (inhibits) B. The six networks described in regulatory terms are: Model 1: a negative feedback loop with three genes; Model 2: a negative feedback loop with two genes and downstream activation; Model 3: a regulatory chain of three genes; Model 4: a coherent feedforward loop; Model 5: a double input module with AND gate, and Model 6: a double input module with OR gate. (B) The Hill function, used as the cis- regulatory input function in the models, plotted with constant threshold for various Hill coefficients.