ESR1 and breast cancer: Based on the predictive performance of each signature in 31 breast cancer test datasets (Additional file 1: Table S2) and nine estrogen receptor-negative (ER-) subsets, we identified 37 significant signature candidates (Additional file 1: Table S3) capable of robustly predicting breast cancer prognosis as a whole and one signature that predicts prognosis in the ER- setting (Table 4).