PCNA and cancer: MCM2 is a sensitive biomarker for cancer cell proliferation in keeping with classical markers proliferating cell nuclear antigen (PCNA) and Ki-67, and performs even better in colorectal cancer (Hanna-Morris et al. 2009), breast cancer (Yousef et al. 2017; Joshi et al. 2015), and esophageal squamous cell carcinoma (ESCC) (Kato et al. 2003).