We have investigated the gene expression of PCNA (Proliferating Cell Nuclear Antigen)/cyclin in rat tissues and the R3230AC mammary tumor. The steady-state mRNA level of PCNA/cyclin in a tissue is related to the proliferation of the tissue. The observation was confirmed with the results from the studies of the immunoblotting analyses and the DNA polymerase activity measurements. Furthermore, an overexpression of PCNA/cyclin was found in the R3230AC mammary tumor, which is accompanied by an altered PCNA/cyclin gene structure detected with the Southern blot analysis.