Complex sentence: Cells enter the G 0 phase from a G 1 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.
Split into simpler sentences: Cells enter the G 0 phase from a cell cycle checkpoint in the G 1 phase, such as the restriction point (animal cells) or the start point (yeast). This usually occurs in response to a lack of growth factors or nutrients.