p21 is a new discovered cell cycle control element, a CDK inhibitor. p15, p16,p27 are also CDK inhibitors. They can bind with cyclin-CDK complexes and inhibit their kinase activity. These CDK inhibitors play roles in G₁ restriction point and G₁/S checkpoint. Furthermore, p21 gene can be regulated by wild type p53. p21 plays a role in p53 mediated G₁ arrest which is induced by DNA damage. Expression of p21 gene is 10～20 times higher in senescence cells than in proliferating cells. It begins to express when muscle cells start differentiation. These results demonstrate that p21 plays a major role in cell proliferation, differentiation and senescence.