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Yichao Liang,1,* Xinling Cai,2,* Xu Zheng,2 Hongzhuan Yin1 1Department of General Surgery, Shengjing Hospital of China Medical University, Shenyang, Liaoning, People’s Republic of China; 2Department of Clinical Oncology, Shengjing Hospital of China Medical University, Shenyang, Liaoning, People’s Republic of China*These authors contributed equally to this workCorrespondence: Hongzhuan YinDepartment of General Surgery, Shengjing Hospital of China Medical University, Shenyang, Liaoning, People’s Republic of ChinaTel +86 18940256969Fax +86 2496615-10094Email chad156@hotmail.comPurpose: To investigate the clinicopathological characteristics of stage I–III colorectal cancer (CRC) patients with deficient mismatch repair (dMMR) protein.Patients and Methods: A retrospective analysis of 61 patients with stage I–III CRC confirmed by immunohistochemistry as dMMR after radical resection at Shenjing Hospital of China Medical University from May 2017 to June 2019 was performed. A total of 183 stage I–III CRC patients with proficient mismatch repair (pMMR) protein from the same period were randomly selected as a control group. The clinicopathological data of the two groups were investigated.Results: There were significant differences between the two groups in age, sex, site of onset, maximum diameter of tumor, T stage, tumor differentiation, and histological type (P < 0.05). No significant difference was detected in nerve vessel invasion, cancer nodules, the N stage or the TNM stage. In the dMMR group, 41 patients (66.13%) showed PMS2/MLH1 deletion, and the number of MSH2/MSH6 deletion is 21 patients (33.87%). Among them, 34 patients (54.84%) had PMS2 and MLH1 deficiency. In total, 16 patients (25.81%) had MSH2 and MSH6 deficiency. A total of 5 patients (8.06%) showed simply PMS2 deletion and 5 patients (8.06%) showed simply MSH6 deletion. In total, 2 patients (3.23%) showed concurrent loss of PMS2, MLH1 and MSH2. No significant difference were found (P > 0.05) in the above factors among dMMR CRC patients with different MMR proteins deletions.Conclusion: Our results show that dMMR status may be more likely exist in female and younger (≤ 55 years) patients with a greater tumor burden (> 5cm), right colon, T4 stage disease, poor differentiation and mucinous adenocarcinoma. Loss of PMS2 and MLH1 is the most common pattern of MMR protein expression deficiency, followed by concurrent deletion of MSH2 and MSH6.Keywords: deficient mismatch repair, clinicopathological characteristics, colorectal cancer, microsatellite instability