{"type":"root","children":[{"type":"paragraph","children":[{"type":"text","value":"p21 is a potent cyclin-dependent kinase inhibitor. The p21 protein binds to and inhibits the activity of cyclin-CDK2 or -CDK1 complexes, and thus functions as a regulator of cell cycle progression at G1. The expression of this gene is tightly controlled by the tumor suppressor protein p53, through which this protein mediates the p53-dependent cell cycle G1 phase arrest in response to a varity of stimuli. In addition to growth arrest, p21 can mediate cellular senescence. p21 can also interact with proliferating cell nuclear antigen (PCNA) and plays a regulatory role in S phase DNA replication and DNA damage repair. Normal cells typically show intense nuclear p21 expression. According to literature, loss of p21 expression has been associated with poor prognosis in several carcinomas."}]}]}