The maintenance of genetic information in a cell is ensured by essential macromolecular events called DNA replication and DNA repair. The cell possesses many control mechanisms that allow the optimal strategy under extreme physiological conditions. A cell that is in the process of replicating its DNA and faces a genotoxic stress such as UV-damage, X-ray damage and influences of mutagens will turn off DNA replication and instead turn on DNA repair mechanisms. We have learned in the last three years that in all these processes identical DNA synthetic enzymes are involved. They are, however, coordinated to the actually required metabolic events through a ring-shaped protein called proliferating cell nuclear antigen (PCNA). Exact knowledge of these protein-protein interactions will eventually open up novel therapeutic strategies against cancer.