The G₀ phase, G zero phase, or resting phase is a period in the cell cycle in which cells exist in a state. G₀ phase is viewed as either an extended G₁ phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle. Some types of cells, such as nerve and heart muscle cells, become quiescent when they reach maturity (i.e., when they are terminally differentiated) but continue to perform their main functions for the rest of the organism's life. Multinucleated muscle cells that do not undergo cytokinesis are also often considered to be in the G₀ stage. On occasion, a distinction in terms is made between a G₀ cell and a 'quiescent' cell (e.g., heart muscle cells and neurons), which will never enter the G₁ phase, whereas other G₀ cells may.