Gene expression signatures suggest that a “proliferation cluster” including Ki-67, proliferating cell nuclear antigen (PCNA), some proliferation-related genes and cell cycle genes may be the strongest predictor for metastasis and relapse in tamoxifen treated ER-positive breast cancer patients, emphasizing the important roles of proliferation genes in prognosis [13], [14], [15]. This evidence concerns the gene PCNA and breast cancer.