The G0 stage, also known as the quiescent phase, is a cellular state where cells exit the active cell cycle and enter a non-dividing but metabolically active state.

Cells enter the G0 phase (also called the quiescent phase) when they are not actively dividing or preparing to divide. This happens for various reasons, including the absence of signals to divide, the completion of their specific function, or when they are fully differentiated and no longer need to divide.

Reasons for entering G0:

• Lack of Mitogens : Mitogens are molecules that stimulate cell division. If these are not present, cells may exit the active cell cycle and enter G0. 

• Cellular Differentiation : Many cells, like neurons and muscle cells, become specialized and fully functional. They exit the cell cycle and enter a terminal G0 phase, where they don't divide further. 

• Cellular Repair : Some cells may enter G0 to repair damage or undergo maintenance. They may then re-enter the cell cycle later when needed. 

• Maintaining Cell Number : G0 helps regulate cell number in the body. It prevents uncontrolled growth by holding cells in a non-dividing state until needed.