ABSTRACT
A crucial decision in every proliferating cell is the decision to continue with a further round of cell division or to exit the cell cycle and return to the stationary phase. Similarly quiescent cells must make the decision whether to remain in the stationary phase (G0) or to enter into the cell cycle (see Chapter 1). Entry into the cycle occurs in response to mitogenic signals and exit in response to withdrawal of these signals. To ensure that DNA replication is complete and that any damaged DNA is repaired, cells must pass through specific checkpoints. Tumor cells undergo uncontrolled proliferation either due to mutations in the signal transduction pathways as described in Chapters 2 and 3 or because of mutations in the regulatory mechanism of the cell cycle.
