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Plot of 
and 
versus the concentration of X for a simple two step pathway with negative feedback. Two perturbations in 
that determines 
are shown. In panel (a), where the feedback is strong, changes in 
have hardly any effect on X. In the panel (b), the same change in 
results in a much larger change in X. This illustrates the homeostatic effect of negative feedback. (a) 
and (b) 
. (Online version in colour.)
