PCNA and gastric cancer: ψ-taraxasterol (1 μM) blocks the cell cycle of gastric cancer cells at the G0/G1 phase through downregulating the expression of cyclin D1 and proliferating cell nuclear antigen (PCNA) and upregulating the expression of p21, thereby inhibiting the proliferation of HGC-27 and NCI-N87 cells [37].