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Nonlinearities of rate laws describing biochemical reaction kinetics can often results in ultrasensitive switches in which small change or fluctuation of parameter can lead to large change in network output. Such switches are important for making robust cell decision choices but can be detrimental for networks functioning in homeostasis and desiring noise minimization. In this presentation I'll discuss biological examples illustrating each of these cases. With combination of mathematical modeling and bioinformatic data analysis, we show that noise minimization and avoidance of ultrasensitive switches explain operon organization of E. coli. These results suggest a central role for gene expression noise in selecting for or against maintaining operons in bacterial chromosomes thereby providing an example of how the architecture of post-translational networks affects bacterial evolution. With combination of mathematical modeling and single-cell microscopy, we show the existence and origins of ultrasensitivity in the network responsible for cell-fate decision in sporulating B. subtilis. These results illustrate how unique structure of the sporulation network allows fast and robust population level response despite cellular variability.