ESR1 and breast cancer: In addition, in order to determine whether the expression of Sox2 has prognostic potential in breast cancer patients treated with tamoxifen, we analysed publicly available patient data sets (GSE9893, GSE12093 and GSE1379) where ER-positive patients (n = 154, 132 and 54, respectively) had been treated with tamoxifen therapy and a minimum of 5-year follow-up data are available.