Effects of noise. Three independent stochastic runs using the Gillespie algorithm of the model's response to DNA damage. Model parameters are changed at time zero as in Fig. 2. Later peaks in p53 level show more variability than earlier peaks. Molecule numbers were chosen small enough to achieve a visible amount of noise. Using realistic numbers makes the stochastic simulations indistinguishable from deterministic behaviour (see text and Figure S6 in additional file 2).