The
G0 phase,
G zero phase, or resting phase is a period in the
cell cycle in which cells exist in a state. G
0 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 G
0 stage. On occasion, a distinction in terms is made between a G
0 cell and a 'quiescent' cell (e.g., heart muscle cells and neurons), which will never enter the G
1 phase, whereas other G
0 cells may.