The immunohistochemistry process was conducted by pathologists in the Department of Pathology of the two Hospitals. The paraffin-embedded CRC tissue was cut into 4-μm thickness sections. The paraffin sections were performed immunohistochemistry detection successively as dewaxed, antigen retrieval, block endogenous peroxidase, serum blocking, primary antibodies incubation, horseradish peroxidase (HRP)-conjugated secondary antibody incubation, 3,3N-Diaminobenzidine staining, counterstained by haematoxylin, graded dehydration and mounting as previous described (Zeng et al., 2018 (link)). The primary antibodies for MMR proteins were ready-to-use antibodies: MLH1 (ZM-0154, Zhongshan Goldenbridge Biotechnology Beijing, P.R. China), MSH2 (ZA-0622, Zhongshan Goldenbridge Biotechnology), MSH6 (ZA-0541, Zhongshan Goldenbridge Biotechnology) and PMS2 (ZA-0542, Zhongshan Goldenbridge Biotechnology). The polymer HRP detection system (PV-9000, Zhongshan Goldenbridge Biotechnology) was used to detect the antigen–antibody binding reaction according to the manufacturer’s protocol (Zeng et al., 2018 (link)). The staining scoring criterion of the grading of staining intensity and the percentage of positive cells was adopted as previously mentioned (Sun et al., 2014 (link)). The results were evaluated by two independent pathologists with anonymous patient’s information.