G0 phase

The G₀ phase (G zero) is a period in the cell cycle where cells exist in a quiescent state. Cells enter the G₀ phase from a G₁ phase checkpoint, such as the restriction point (animal cells) or the start point (yeast) usually in response to a lack of growth factors or nutrients. During the G₀ phase, the cell-cycle machinery is dismantled and cyclins and cyclin-dependent kinases disappear. Some cells remain in the G₀ phase until there is a reason for them to divide. For instance, epidermal fibroblasts remain in the G₀ phase until stimulated by growth factors generated in response to a wound.

Although many cells in the G₀ phase may die along with the organism, this does not mean that cells that enter the G₀ phase are destined to die; this is simply a consequence of the cell lacking any stimulation to re-enter in the cell-cycle.