The protein p21WAF1/Cip1
inhibits the activity of several cyclin/cyclin-dependent kinase complexes and blocks cell-cycle progression (1). In tumor cells that have lost the p53 protein, or contain an altered form of p53, p21WAF1/Cip1
levels are dramatically reduced or totally absent (2). Because p21WAF1/Cip1
appears to mediate several of the growth-regulatory functions of p53, its expression would be predicted to reflect the functional status of p53 more precisely than p53 accumulation.