ESR1 and breast cancer: Most of the estrogen-receptor-negative tumors are classified as basal-like tumors according to the molecular subtype, and estrogen-receptor-positive tumors are mostly of the luminal subtype, luminal A and B. Since the METABRIC cohort was the largest of the four cohorts in this study, it was chosen to analyze breast-cancer-specific survival (BCSS) according to estrogen receptor and molecular subtype status.