Identification of the cellular proteins whose expression is regulated during the cell cycle in normal cells is essential for understanding the mechanisms involved in the control of cell proliferation. A nuclear protein called cyclin of relative molecular mass 36,000 (Mr 36K), whose synthesis correlates with the proliferative state of the cell, has been identified in several cell types of human, mouse, hamster and avian origin1-5. The rate of cyclin synthesis is very low in quiescent cells and increases several fold after serum stimulation shortly before DNA synthesis6,7. Immunofluorescence and autoradiography studies have shown that the nuclear staining patterns of cyclin during S phase have a sequential order of appearance and a clear correlation can be found between DNA synthesis and cyclin positive nuclei7-10. The proliferating cell nuclear antigen (PCNA)8,11-13 and cyclin have many common properties and it has been shown that these two are identical12,14. Recently a protein which is required by DNA polymerase-δ for its catalytic activity with templates having low primer/template ratios has been isolated from calf thymus15. We report here that cyclin and the auxiliary protein of DNA polymerase-δ are identical.