<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 because it's part of their internal genetic programming . </P>

Why do nerve and muscle cells leave the cell cycle