S curves of Figure 1 and 2. In the simulation, we adopted the same doses of both inhibitors like . The left subfigure is the simulation results of the serial structure in Figure 1. The right subfigure is the simulation results of the parallel structure in Figure 2. The S curves of original systems are in red. The blue curves are the S curves recovered from the results of simplified systems based on Lemma 1 and the two corollaries. In fact the S curve of the simplified serial system has opposite sign to that of the original serial system. Obviously the S curves recovered agreed with the original S curves. The signs of S curves are negative showing the synergistic effect of the serial and parallel structures. In left subfigure the absolute values of S are much small (10-3 order) to those of the S in right subfigure (10-1 order). Combination therapy of the parallel structure could be more effective. These results are conincident with the results in .