Growth arrest or G₀ is viewed as either an extended G1 phase, where the cell stops growing (in M phase) due to mutation/damage, due to contact inhibition (crowding), or lack of food.

Not all cells adhere to the normal cell cycle pattern in which a newly-formed daughter cell immediately enters the interphase, closely followed by the mitotic phase. Cells in G₀ phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle. Some cells enter G₀ temporarily until an external signal triggers the onset of G₁. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G₀ permanently.