G0

G0 is a state in the eukaryotic cell cycle in which cells are metabolically active but not actively preparing to divide. It is commonly referred to as a resting or quiescent phase, though it is not a fixed part of the canonical sequence G1–S–G2–M. Cells may enter G0 from G1 after growth factor withdrawal, nutrient limitation, contact inhibition, or stress, and they may remain there for variable lengths of time.

In multicellular organisms, G0 is especially associated with mature, differentiated cells that do not routinely divide, such as most neurons and some muscle cells. Other cell types, including many fibroblasts and hematopoietic cells, can temporarily exit the cell cycle into a G0-like state and later re-enter G1 when conditions become favorable. Some cells can permanently reside in a deep G0 or enter senescence, a state of irreversible growth arrest often accompanied by changes in gene expression and morphology.

Molecularly, G0 is characterized by low activity of cyclin-dependent kinases, a hypophosphorylated retinoblastoma (Rb) protein, and repression of E2F target genes. Cells in G0 typically express CDK inhibitors such as p27Kip1 and p21Cip1. Re-entry into the cycle requires mitogenic signaling that induces Cyclin D-CDK4/6 activity, Rb phosphorylation, and activation of E2F-driven transcription to initiate S-phase entry.

G0 is a reversible departure from the proliferative cycle in many contexts, but the boundary with irreversible states such as senescence can vary by cell type and context.