The restriction point and control of cell proliferation

Research over the past two decades has defined a window of time in the early/mid G(1) phase of the cell cycle during which mammalian cells are responsive to extracellular signals. Recent evidence indicates that this period ends with the phosphorylation of the retinoblastoma protein, enabling the cells to pass through the restriction point al the end of mid G(1) phase and to commit to completing the remaining phases of the growth cycle.