Final answer:
Cells can exit the cell cycle before the G1 restriction point and enter G0 phase, which is a state of inactivity. The G1 checkpoint assesses whether conditions are right for division, and cells that don't meet these conditions may go into G0 temporarily or permanently.
Step-by-step explanation:
Cells may leave the cell cycle before reaching the restriction point late in G1, and enter G0 phase. The G1 checkpoint is critical in determining if a cell will proceed with the division process. Cells that do not meet the necessary conditions for division, such as adequate reserves, correct cell size, and no genomic DNA damage, might enter the G0 phase. This phase is a quiescent (inactive) stage either temporarily, until conditions improve, or permanently for cells that rarely divide like mature cardiac muscle and nerve cells.