PCNA and esophageal squamous cell carcinoma: To further investigate the potential mechanism(s) by which MT3‐MMP negatively regulates tumorigenesis and aggressiveness of ESCC, Western blot analysis was conducted to monitor expression of multiple key cell cycle‐regulatory proteins, including p21Cip1, p27Kip1, cyclin A, cyclin D, cyclin B1, cyclin E, MCM, Rb, and PCNA.