PCNA, originally known as Cyclin, is a 36kD nuclear protein associated with the cell cycle. It has recently been identified as the polymerase delta accessory protein. PCNA is essential for cellular DNA synthesis. It is also required for the in vitro replication of simian virus 40 (SV 40) DNA, where it acts to coordinate reading and lagging strand synthesis at the replication fork. This antibody is a useful tool for studying the proliferating cells in normal tissues, and possible aberrant expression in neoplasm. It is useful for staining the nuclei in proliferating calls during the G1, S, G2, and M stages. It has been suggested that high expression of PCNA correlates with organ metastasis and vascular invasion, and may be an effective prognostic factor (2).