Network motifs. a) Single input motif (SIM), b) regulatory chain motif (RC), c) feed forward motif (FF), and d) feedback motif (FB). The abbreviations are: S, input signal; y and z, monitored variables; α, β reaction rates. Solid arrows denote production and degradation reactions, while dashed arrows denote control mechanisms. Normal arrowheads denote activation, while flat arrowheads denote inhibition.