The earlier work on the structures of the processivity factors of DNA polymerases (E. coli beta subunit and Yeast PCNA) was continued by determining the structure of the human DNA polymerase clamp (PCNA) in complex with an inhibitor of the cell cycle. This inhibitor, known as p21WAF1/CIP1, has two functions. One part of the protein blocks the action of the cyclin-dependent protein kinases, while the other part blocks the ability of eukaryotic DNA polymerases to bind to the ring-shaped DNA clamp. This paper describes how the p21WAF1/CIP1 protein wraps around the surface of the PCNA ring. Subsequent work from the Steitz and Kuriyan groups established, respectively, that the DNA polymerase catalytic subunit as well as the DNA polymerase clamp loaders (which open the ring-shaped processivity factors and load them onto DNA) both attach to PCNA using the same region that is blocked by p21WAF1/CIP1.