# Single Input Module

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The genes $Z_i$ recieve a single input from the master transcription factor $Y$ that is usually autoregulated. The order of activation of the $Z_i$s can be tuned by the respective activation threshold $K_{yi}$. In an enzyme pathway that produces a desired product it is convinient to first start producing the first enzyme of the pathway, then the second, thrid ans so on. Thus the according activation thresholds of the enzyme coding genes are tuned to satisfy $K_{yi-1} < K_{yi} < K_{yi+1}$. The activation threshold increases sucessively if one follows the pathway downstream.