Control of the G1/S phase transition of the cell cycle contributes to the maintenance of homeostasis. It is well known that the disruption of the cell cycle is related to cell transformation and carcinogenesis. The G1/S phase transition involves a network of components that includes cyclins, cyclin-dependent kinases, and other proteins. Numerical simulation techniques and system analysis are expected to become powerful tools for investigating complex biological networks. To reach this goal, we designed a mathematical model of the G1/S phase transition. Using our model, we conducted a numerical simulation and comprehensive system analyses of this phase of the cell cycle. In this way, we were able to predict the key factors involved in the control of the G1/S transition.