Expression of proliferating cell nuclear antigen (PCNA) or cyclin or polymerase δ auxiliary protein is elevated in the nucleus during late G1 phase immediately before the onset of DNA synthesis, becoming maximal during S-phase and declining during G2 and M phases. PCNA/cyclin may act as an auxiliary protein of DNA polymerase-δ to play a fundamental role in the initiation of cell proliferation. Recombinant PCNA was expressed with a 6x His tag in sf9 insect cells and purified by proprietary chromatographic techniques. MW: 29.8 kDa. Purity ≥90%