Figure 1. Key variables and system outputs explored in mathematical models. (A) The number of kinases present within the module: one kinase = one-Tiered (1T), 2 kinases = 2-tiered (2T), 3 kinases = 3-tiered (3T) and 4 kinases = 4-tiered (4T). (B) Two modes of activating kinases in the MAPK module: processive phosphorylation [P] (upper panel), where phosphorylation sites within the activation loops are phosphorylated during a single binding step: vs. distributive phosphorylation [D] (lower panel), where only one phosphorylation occurs during binding, and the kinases must re-bind for the second phosphorylation to occur. (C) The 4 different outputs that can be generated from MAPK modules. Classic Michaelis–Menten (MM) kinetics⁷² where the system is initially linear but then becomes saturate; analog output, which transmit continuous information that is directly proportional to the input stimulus¹⁷; switch-like output, which follows a sigmoidal dose-response curve where initially the system is relatively unresponsive and then responds rapidly to input^10,73; and digital output, where the system can only stably exists in one of 2 states, off or on¹⁵ with a threshold that defines the switch.