Solution: In mature animals, certain cells will not divide, and many others will divide only when it is necessary to replace cells that have been lost due to injury or cell death. These cells depart the G1 phase and enter the G0 phase of the cell cycle, which is a dormant stage of the cell cycle. G0 cells do not multiply unless they are stimulated to do so. As a result, cells in this phase become passive, cease dividing, and specialize as a result of the differentiation process.