Objective To explore the relationship between the expression of MutS homolog2 (MSH2) and CyclinD1 proteins in breast cancer and their clinical significance.Methods Expression of MSH2 and CyclinD1 proteins was detected in 67 cases of invasive ductal carcinoma of the breast by immunohistochemistry.Results The positive expression rate of MSH2 and CyclinD1 proteins was 53.7％ (36/67)and 47.8％ (32/67)respectively.The expression of MSH2 and CyclinD1 protein was closely related to lymph nodes metastasis(P ＜ 0.05).Expression of MSH2 and CyclinD1 protein was negatively correlated with each other in breast cancer(r =-0.251,P =0.040).Conclusion CyclinD1 may play a role in DNA mismatch repair(MMR) system by downregulating the expression of MSH2 and thus influences the tumorigenesis and progression of breast cancer.