Expression of proliferating cell nuclear antigen (PCNA) or cyclin or polymerase delta auxiliary protein is elevated in the nucleus during late G1 phase immediately before the on set of DNA synthesis, becoming maximal during S-phase and declining during G2 and M phases. Its level correlates directly with rates of cellular proliferation and DNA synthesis. PCNA/cyclin may act as an auxiliary protein of DNA polymerase-delta to play a fundamental role in the initiation of cell proliferation.