Parameter robustness test of the models.

<p>We tested how extrinsic parameter variations in the regulation of the transcriptionally controlled proteins influence the timing of cell cycle transitions. The parameters that control synthesis and degradation of the activator (<b>A</b>) or inhibitor (<b>B</b>) were randomly sampled (1000 parameter sets) between one tenth and ten times the basal values and the variations in the timing of the transitions are reported versus a measure of parameter variation distance as earlier defined <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029716#pone.0029716-Barkai1" target="_blank">[68]</a>. Each colored dot represents the average of 100 parallel stochastic simulations at a randomly drawn parameter set, orange dots stand for parameter combinations where not all 100 simulations gave successful transitions (TR* hitting the critical value). Connected blue dots give the average percentage of successful transitions, with black lines giving ± standard deviation (corresponding values on the right y-axis).