Stationary model validation: insulin binding and receptor phosphorylation. Left: Cell-associated radioactively labeled insulin is shown as a function of the stationary insulin concentration and compared to experimental data (Figure 4 A in ). Almost no labeled insulin should bind to receptors at maximal concentrations of unlabeled insulin. Therefore, the value for the highest concentration of unlabeled insulin was treated as background and subtracted from all values. Right: The fraction of phosphorylated receptors is shown as a function of the stationary insulin concentration and compared to experimental data for receptor activation (Figure 4 B in ). We regard receptor phosphorylation as a good indicator for receptor activity.