Layer-based reaction network of the example system. The receptor layer (left) describes the processes of ligand binding and receptor phosphorylation by two reactions each. The effector layer (right) describes the effector binding process by one reaction. The signals between the layers are the sum of species with a phosphorylated binding site x and the sum of species with an occupied binding site xb (which in this case is only the species RXE). Black arrows indicate reactions, green arrows indicate signal flows.