Simplification of the PPX part leads to model M4. The same notation as in Figure 3 is used. Additionally, a red colored parameter name indicates, the introduction of a new parameter. In this simplification step PPX and all corresponding reactions have been removed from the model. In order to still account for STAT1cPhos_2 dissociation and phosphorylation, we added a new dissociation reaction containing the parameter k11new. The mathematical description of model M4 is given in Additional file 1 table S6a and S6b.