Background Estrogen receptor alpha(ER a)is the most important endocrine therapy responsiveness predictor for women with breast cancer.The accuracy of the prediction of the response to endocrine therapy was thought to be affected by involving the estrogen receptor coregulatory proteins and cross-talk between ER and other growth factor-signaling networks.Nuclear corepressor 1(NCOR1)is one of the ERa transcription repressor.The objective of the study is to investigate the expression of NCOR1 at the protein level and pursue its predictive value for breast cancer endocrine therapy.Methods In the present study,the level of expression of NCOR1 protein has been assessed by immunohistochemistry in 104 cases of invasive carcinoma of the breast.Associations between NCOR1 protein expression and different clinicopathological factors and survival were sought.Results It was found that NCOR1 was expressed at significantly higher levels in responsive patients treated with endocrine therapy as first-line treatment on relapse.Responsive patients also had a significantly longer post-relapse survival and overall survival.No NCOR1 expression difference was found between patient by age,tumor size,lymph node status,different histological grade groups and human epidermal growth factor receptor 2(HER2)status.Multivariate analysis showed that NCOR1 is an independent prognostic factor for over-all survival.Conclusions In breast cancer,NCOR1 protein expression level predicts response to endocrine therapy as first-line treatment for breast cancer patients on relapse and NCOR1 protein level assay may increase the accuracy in the endocrine treatment determination and,therefore,improving the patients survival.