What factors are important for maintaining quiescence?

Quiescent cells are characterized by the lack of expression of cell cycle genes, and the down regulation of mRNA production and protein synthesis. Cells in this stage do not actively proliferate, but are able to do so if necessary. To maintain a quiescent state, these cells are regulated by cell cycle and transcriptional regulators.

• Cyclin-dependent kinases (CDKs) help progress the cell cycle. CDK inhibitors, such as p27, p21 and p53, play a vital role in inhibiting CDK activity and preventing cell cycle progression.
• Members of the Myc/Mad/Max network regulate expression of genes involved in cell cycle progression and cellular growth. Their suppression is associated with quiescence. 
• Quiescent cells also have mechanisms to monitor and respond to DNA damage. The DNA damage response pathway involving proteins such as p53 and ATM/ATR kinases, detects DNA lesions and triggers cell cycle arrest, or apoptosis, to prevent DNA damage.
• Mitogenic signaling pathways that promote cell proliferation are downregulated. IGF and MAPK pathways are inhibited to prevent cells from receiving pro-proliferative signals. 
• Epigenetic regulators such as DNA methylation, histone modification also contribute to the maintenance of quiescence; they do this by silencing genes associated with proliferation.