<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2013) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2013) (Learn how and when to remove this template message) </Td> </Tr> <P> The G phase describes a cellular state outside of the replicative cell cycle . Classically, cells were thought to enter G primarily due to environmental factors, like nutrient deprivation, that limited the resources necessary for proliferation . Thus it was thought of as a resting phase . G is now known to take different forms and occur for multiple reasons . For example, most adult neuronal cells, among the most metabolically active cells in the body, are fully differentiated and reside in a terminal G phase . Neurons reside in this state, not because of stochastic or limited nutrient supply, but as a part of their internal genetic programming . </P> <P> G was first suggested as a cell state based on early cell cycle studies . When the first studies defined the four phases of the cell cycle using radioactive labeling techniques, it was discovered that not all cells in a population proliferate at similar rates . A population's "growth fraction"--or the fraction of the population that was growing--was actively proliferating, but other cells existed in a non-proliferative state . Some of these non-proliferating cells could respond to extrinsic stimuli and proliferate by re-entering the cell cycle . Early contrasting views either considered non-proliferating cells to simply be in an extended G phase or in a cell cycle phase distinct from G--termed G. Subsequent research pointed to a restriction point (R - point) in G where cells can enter G before the R - point but are committed to mitosis after the R - point . These early studies provided evidence for the existence of a G state to which access is restricted . </P>

Where does g0 occur in the cell cycle
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