PCNA (Proliferating Cell Nuclear Antigen), plays a central role in coordinating the association of replication factors during DNA replication as well as during recognition and repair of DNA damage. PCNA forms a toroidal, ring-shaped structure of 90 kD by the symmetric association of three identical monomers. The ring encircles the DNA, acts as a platform upon which polymerases and other proteins dock to perform various DNA metabolic processes, and functions as a DNA polymerase-delta co-factor. The association of proteins with the replication fork through interactions with PCNA is regulated by several mechanisms. PCNA interacts with numerous proteins to facilitate DNA replication and repair, epigenetic processes and sister chromatid cohesion. PCNA has its highest expression during G1and S-phases, and its expression decreases in G2 and M-phases. This marker is also present in the early G0 phase because of its long half-live of 8-20 h. PCNA is a nuclear nonhistone protein that is necessary for DNA synthesis and is an accessory protein for DNA polymerase alpha, which is elevated during the G1/S phase of the cell cycle. Several studies have been performed to evaluate cell proliferation using PCNA and Ki-67 in different tumors of various origins; compared with PCNA, Ki-67 has been shown to be more sensitive and specific in the various tumors.