THE protein p21 (WAF1, CIP1 or sdil), induced by the tumour-suppressor protein p53, interacts with and inhibits two different targets essential for cell-cycle progression^1–8. One of these is the cyclin–Cdk family of kinases and the other is the essential DNA replication factor, proliferating-cell nuclear antigen (PCNA). We report here that separate domains of p21 are responsible for interacting with and inhibiting the two targets. An amino-terminal domain inhibits cyclin–Cdk kinases and a carboxyl-terminal domain inhibits PCNA. Using these separated domains, we have determined that p21 inhibits different biological systems through different targets. The PCNA-binding domain is sufficient for inhibition of DNA replication based on simian virus 40, whereas the Cdk2-binding domain is sufficient for inhibition of DNA replication based on Xenopus egg extract and for growth suppression in transformed human cells.