CDKN1A (p21,CIP1,WAF1),a cyclin-dependent kinase inhibitor,is necessary for proper control of the cell cycle and premature senescence. P21 is a cyclin-dependent kinase (CDK) inhibitor that suppresses proliferation by inhibiting CDK2 and CDK1 activity at the G1/S and G2/M transitions. P21 is a major mediator of p53 to induce cell cycle arrest in G1. p21 can interact with proliferating cell nuclear antigen (PCNA),and plays a regulatory role in S phase DNA replication and DNA damage repair. P21 was reported to be specifically cleaved by CASP3-like caspases,which thus leads to a dramatic activation of CDK2,and may be instrumental in the execution of apoptosis following caspase activation. Overexpression of p21 decreases the expression levels of cell cycle progression genes and upregulates senescence-inducing genes.