Table 1.

Effect of removing mass, to mimic retroactivity, from a double cycle on the sensitivity of APP with respect to signal S. The model uses linear mass-action kinetics so that the observed ultrasensitivity is due entirely to the first-order effects. Note the first-order ultrasensitivity remains unchanged although the gain, as defined by d APP/dS, decreases because the total mass of the system decreases.

total mass in cycle	concentration of APP	sensitivity of APP to signal: $C_S^{\mathrm{APP}}$	gain (dAPP/dS)
5.0	0.6397	1.6820	1.076
1.0	0.1279	1.6820	0.215
0.9	0.1151	1.6820	0.194
0.4	0.0511	1.6820	0.086