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G0 phase

Non-proliferating (not dividing) cells are in the resting phase G0, where they may remain for a few days or up to several years. Under specific conditions, they may re-enter the cell cycle. However, cells in the G0 phase are often terminally differentiated and carry out their function until cell death without ever dividing again. For example, most lymphocytes in human blood are in the G0 phase and remain there unless they are stimulated by a specific antigen to re-enter the cell cycle via the G1 phase. Through a lack of better knowledge, this phase was previously ascribed the function of cell reservoir; roles in secretion and defense against pathogens are also hypothesized. G0 phase cells are not completely dormant, continuing to carry out essential cell functions in reduced form. A majority of the variability in time between cell divisions in different tissues arises from G0 phase cells. Nerve and skeletal muscles thus rarely divide, while intestinal cells divide every 20 minutes. The continuance of cells in the G0 phase is not caused by the absence of a mitosis signal; instead it results from the active repression of the mitosis gene. Defective repression is a further mechanism for the occurrence of cancer. Some experts dispute the existence of the G0 phase.