Cyclin, also known as PCNA or the auxiliary protein of mammalian DNA polymerase delta, is a stable cell cycle regulated (synthesized mainly in S-phase) nuclear protein of apparent Mr 36,000 whose rate of synthesis correlates directly with the proliferative state of normal cultured cells and tissues. Cyclin (PCNA) is absent or present in very low amounts in normal non-dividing cells and tissues, but it is synthesized in variable amounts by proliferating cells of both normal and transformed origin. All available information indicates that this ubiquitous and tightly regulated DNA replication protein is a central component of the pathway(s) leading to DNA replication and cell division.