ObjectiveTo explore expression of Mdm2 in the estrogen receptor α (ERα)-positive breast cancer tissues and fibroadenoma of breast tissues, and to explore the effect of MDM2-siRNA on cell proliferation, colony formation, and apoptosis for MCF-7 cells. Methods① Seventy eight ERα-positive breast cancer patients identified by histopathological examination, who underwent surgery in our hospital from June 2012 to October 2015, as well as 10 fibroadenoma of breast patients underwent surgery in the same period, were collected retrospectively to determine the expression of Mdm2, then explore the relationship between the expression of Mdm2 and clinical pathological characteristics of ERα-positive breast cancer patients. ② MCF-7 cells were divided to MDM2-siRNA group (added with MDM2-siRNA), negative control group (added with negative siRNA), and blank control group (added without any reagent). Expression of Mdm2, cell proliferation rate, number of colony formation, and apoptosis rate were determined in the MCF-7 cells of 3 groups. Results① No one of fibroadenoma of breast patients was found positive expression of Mdm2 (0/10), and 38 of 78 ERα-positive breast cancer patients were found the positive expression of Mdm2 (48.7%), which is higher than that of fibroadenoma of breast tissues (χ2=12.357, P=0.000). In ERα-positive breast cancer patients, expression of Mdm2 was related with TNM staging and number of metastasic lymph node (P < 0.050), the positive expression rate of Mdm2 was higher in patients with later TNM staging or more metastasic lymph node. ② Cell proliferation rates on 2, 3, and 4 days after transfection, expression level of Mdm2, and number of colony formation were all lower (P < 0.050), but the apoptosis rate was higher in MDM2-siRNA group (P < 0.050), comparing with negative control group and blank control group. But there was no significant difference between negative control group and blank control group on aforementioned indexes (P > 0.050). ConclusionMdm2 is a diagnostic marker in ERα-positive breast cancer patients, and treatment targeting it might has a certain therapeutic value.