Combinatorial signalling and specificity. Combinatorial signalling can increase signalling specificity through competitive inhibition of parallel pathways. This figure depicts the outputs of the A and B pathways when subject (at t = 0) to a change in production rate of X, starting from zero. The concentrations of the active output of pathways A and B are given by black and grey curves, respectively. Solid curves represent the response to both X and Y together, while dashed curves represent the response to X alone. a) shows the response when X binding to A precedes Y binding, b) shows the response when Y binding to A precedes X binding, c) shows how the amount of available Y affects the fraction of free X and d) shows the response when X and Y bind to A cooperatively. In all cases, it is seen that the presence of X and Y together not only activates the A pathway but also inhibits the B pathway, and thus leads to improved signalling specificity. In (a) and (b), the absence of Y leads to zero output from the A pathway. While the qualitative results from (a) and (b) are similar, we some differences in how the total availability of Y affects the results. In the case where Y binds to A before X binds to it, we see that a low availability of Y substantially increases the fraction of free X. This is not the case when Y binds to A after X is bound to it.