P21/CIP1/CDKN1A is a potent cyclin-dependent kinase inhibitor. It binds to and inhibits the activity of cyclin-cyclin-dependent kinase2 or -cyclin-dependent kinase4 complexes, and thus functions as a regulator of cell cycle progression at G1. Expression of p21/CIP1/CDKN1A is tightly controlled by the tumor suppressor protein p53, and it mediates the p53-dependent cell cycle G1 phase arrest in response to a variety of stress stimuli. p21/CIP1/CDKN1A can interact with proliferating cell nuclear antigen, a DNA polymerase accessory factor, and plays a regulatory role in S phase DNA replication and DNA damage repair. [taken from NCBI Entrez Gene (Gene ID: 1026)].