PCNA and breast cancer: For example, it may enhance protein expression, such as that of ubiquitination [206], activate glycogen synthase kinase-3 beta (GSK3β) [207], increase levels of proliferating cell nuclear antigen (PCNA) and marker of proliferation (Ki-67) [208], and also regulate the cell cycle via the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) pathway [209,210], thereby promoting breast cancer cell proliferation.