Simulations of the cell cycle from figure 7 using a rule-based model. a. The growth rate of cell mass has a controlling effect on the rest of the cycle. It was set to make the cycle length approximately 1 day, a typical value for human cells undergoing mitosis . The shape and ordering of curves for cyclin B, Cdc14 and APC are very similar here to those shown in Tyson's cell cycle model . The cell mass ranges between 1.0 and 2.0. Chromosome separation is a discrete event that is either on or off. For clarity, the curve was shifted up by one unit in the graph. Another discrete event, cell division, is not shown here. b. Tumor growth in this model is controlled directly by the cell cycle. The rule is: tumor growth is incremented whenever cell division occurs. Thus, if the cell cycle stops, tumor growth stops. Tumor growth is also faster when the cell cycle period is shorter. Tumor cell apoptosis occurs at a constant rate. Tumor size has a large range of values, 0 to 100 in arbitrary units, to allow virtually unlimited tumor growth in model runs.