The cell cycle model shown integrates chemical kinetics, enzyme catalyzed reactions and discrete events, such as chromosome separation, cell division, and entry/exit to a combined G1SG2 phase. Tumor growth is a process that is linked explicitly to the cell cycle, thus providing a quantitative model that connects a molecular process model to a clinical manifestation. Each cell division increments tumor size, while a continuous apoptotic reaction causes slow decay of tumor size. Rules for this cell cycle model are given in the text. Simulation results are presented in the succeeding figures.