Reappraisal of serum starvation, the restriction point, G0, and G1 phase arrest points

The restriction point in the G1 phase of the mammalian cell cycle is the oldest, best-known, and widely accepted control point regulating division cycle in mammalian cells. Origins of the restriction point and its subsequent history are reanalyzed here. The initial proposal of the restriction point has an alternative explanation, which is that cells arrested with a G1 phase amount of DNA can arise from the inhibition of a process or processes occurring throughout the cell cycle and are not restricted to any particular phase of the cell cycle or specifically related to any event in the G1 phase of the cell cycle. The initial evidence and subsequent analyses require reexamination. It is proposed that the arrest of cells with a particular DNA content equivalent to that in cells in the G1 phase of the division cycle does not mean there is any particular G1 phase control point.