Cells of all types may enter the G0 phase of the cell cycle. When a cell stops dividing, having gone through what is know as "terminal differentiation", it begins to reside in the gap phase called G0.
While many cells may stop dividing during the lifespan of an organism, they can begin to divide again in certain situations. If, for example, a cell in the G0 phase receives an external signal that stimulates growth, the cell can leave its dormant state. The cycle of duplication and division of cells is called the cell cycle. This cycle constitutes a highly organized and regulated process of cell duplication and division. Distortions within the system are what lead to abnormal development and cancer. With respect to distortions within cells, for example, damaged DNA needs to be repaired before being replicated.