The G0 phase (G sub zero) or the zero of G is a period of the cell in which it remains in a vegetative state. The G0 phase is seen as a distinct and quiet stage that occurs outside the cell cycle. This phase is related to the "Post-Mitotic" state because they are in a non-dividing phase outside of the cell cycle; some cell types (such as neurons and heart muscle cells) when they reach maturity (that is, when they are terminally differentiated) become post-mitotic (enter the G0 phase), and perform their main functions for the rest of the life of the organism. Poly-nucleated muscle cells that do not undergo cytokinesis are often considered G0 phase cells.
