ObjectiveTo analyze the clinicopathologic characteristics and prognosis of human epidermal growth factor receptor 2 (HER2)-negative breast cancer patients with different expression status of estrogen receptor (ER). MethodsThe patients with HER2-negative breast cancer met the inclusion and exclusion criteria and were treated in the Affiliated Hospital of Southwest Medical University from January 1, 2017 to December 31, 2019 were retrospectively collected, and then were assigned into 3 groups according to the ER expression status: ER-negative (ER expression positive rate <1%) group, ER-low expression (ER expression positive rate 1%–10%) group, and ER expression positive rate >10% group. The differences of clinicopathologic characteristics, therapy, and prognosis among the 3 groups were compared. And the risk factors affecting recurrence and metastasis of patients with ER-low expression were analyzed by Cox proportional hazards regression model. ResultsA total of 610 patients with HER2-negative breast cancer were included in this study, including 130 patients in the ER-negative group, 48 patients in the ER-low expression group, and 432 patients in the ER expression positive rate >10% group. The Bonferroni method was used to correct the test level after pairwise comparison, it was found that the histological grade was later (P<0.001, P=0.023) and the Ki-67 expression was higher (P<0.001, P=0.023) in the ER-negative group and ER-low expression group as compared with the ER expression positive rate >10% group; The proportion of the patients receiving chemotherapy in the ER-negative group was higher than that of the ER expression positive rate >10% group (χ2=10.310, P=0.001), while which had no statistical difference between the ER-low expression group and the ER-negative group or the ER expression positive rate >10% group (Fisher exact probability method, P=1.000; χ2= 3.585, P=0.058); The proportion of patients receiving endocrine therapy in the ER-low expression group was higher than that in the ER-negative group (χ2=36.333, P<0.001) and lower than the ER expression positive rate >10% group (χ2=246.996, P<0.001). The difference in disease-free survival (DFS) curves among 3 groups was statistically significant (χ2=46.805, P<0.001); There were no statistical differences in the overall survival (OS) curve and DFS curve between the ER-negative group and the ER-low expression group (Two stage test, P=0.786; χ2=1.141, P=0.286), and which in the ER expression positive rate >10% group were significantly better than thoses in the ER-negative group (χ2=10.137, P=0.001; χ2=39.344, P<0.001) and the ER-low expression group (χ2=4.075, P=0.044; χ2=31.911, P<0.001). The results of multivariate Cox proportional hazards regression analysis showed that N1 and N2 [N0 as reference: RR (95%CI)=7.740 (1.939, 30.897), P=0.004; RR (95%CI)=9.513 (1.990, 45.478), P=0.005) and T3 [T1 as reference: RR (95%CI)=27.357 (2.188, 342.041), P=0.010] increased the probabilities of recurrence and metastasis HER2-negative breast cancer patients with ER-low expression. ConclusionsAccording to results of this study, patients with HER2-negative breast cancer showed certain differences in histological grade and Ki-67 expression among patients with three different ER expression status, but no statistical difference is found between ER-low expression and ER-negative breast cancer, and the prognoses of both are worse than that of ER expression positive rate >10% breast cancer patients. Lymph node metastasis and larger tumor are risk factors affecting recurrence and metastasis in ER-low expression breast cancer patients.