Cell cycle progression is regulated by cyclins and their cognate Cdks. p21 (WAF1/CIP1) inhibits the activity of each member of the cyclin/Cdk family, and over expression of this protein inhibits the proliferation of mammalian cells. The expression of p21 is inducible by a wide range of stress stimuli. p21 is tightly regulated at the transcriptional level by p53, and probably serves as the effector of p53 cell cycle control.