Visualization of the reduced kinetic model. In this small example system the left module is the receptor layer which includes ligand binding and phosphorylation of the binding site on the receptor. The right module is the effector layer, where binding of E to phosphorylated binding sites on R is described. The equations for this system are shown in Equation 26. Note that we could also compute RXp in the receptor layer and transfer it to the effector layer instead of x. The choice to choose x or RXp as a transfer variable is up to the modeler. We, however, always transfer x, as then macroscopic quantities that correspond to experimental readouts are transfered between the layers.