PCNA (Proliferating Cell Nuclear Antigen) was first identified as a factor expressed in the nuclei of replicating cells. PCNA plays a role in both DNA synthesis and repair. PCNA functions as a processivity factor for DNA polymerase by acting as a DNA clamp that tethers the polymerase catalytic units to the DNA template. The structure of PCNA is that of a trimeric ring that encircles and slides along replicating DNA. PCNA interacts with and is regulated by several proteins involved in cell cycle control and check point processes.