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Figure 4 | BMC Systems Biology

Figure 4

From: Specialized or flexible feed-forward loop motifs: a question of topology

Figure 4

Possible shapes of nullcline. The set of possible shapes for the nullcline Ż = 0 with (brown) and without (blue) input. Figures (a), (b) y (c) correspond to different biological regulations. In (a) the joined regulation by proteins X and Y enhances the production of the output Z (βXY> 1). In (b) and (c) the production of Z is reduced (βXY< 1), but in (b) the reduction is smaller due to βXY> βY, whereas in (c) we apply strong inhibition βXY<βY. Insets in (b) show the feasible trajectories for C4 and I1, respectively. For C4 the functionality of the trajectory is uniquely defined as grader, whereas I1 can implement pulser or grader dynamics, depending on the specific parameters.

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