Figure 6.

Factors Contributing to Cell-to-Cell Variability

(A) Evolution of the predicted variances when only some of the model parameters vary from one cell to another: either the parameters that describe the effect of the upstream signals (continuous lines), the parameters controlling the initial conditions (dashed lines), or the reaction rates (dotted lines). The light gray continuous line is the predicted variance when all model parameters differ between cells. The squared dots represent the measured variance of the concentration of the two proteins over the cell populations.

(B) Comparison of the observed (black dots) and the simulated joint distribution of the concentration of ppERK and ppMEK at different time points. The lines indicate the contour of the joint distribution when the system is simulated under the full extrinsic noise model (yellow) or only varying the parameters k₁ and k₁₀ between cells (green).