{"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 variety 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 display a rather intense nuclear p21 expression. Loss of p21 expression has been associated with poor prognosis in several carcinomas including Gastric Carcinoma, Non-Small Cell Lung Carcinoma and Thyroid Carcinoma."}]}]}