Time courses. Hypothetical time courses of the concentration of active protein P* for wild-type (WT) and mutated cells (Mut1, Mut2). The kinetic parameters k1 and k3 are the same for the three configurations (k3/k1 = 0.5). The initial conditions are: P(0) = 1; P* = 0. Figure 5 [a]: response of both mutants when the system is supposed to have only single protein activation. Figure 5 [b]: response of both mutants when a dual mechanism of activation is considered. Figure 5 [c]: response when the mechanism of activation is double protein activation. The configuration of the system for wild-type cells implies an initial intensity for the activated receptor R(0) = 1, while in mutant cells R(0) = 0.5.