ESR1 and breast cancer: Of the breast cancers with absent or very low estrogen receptor (ER) and progesterone receptor (PR), and without overexpression of the human epidermal growth factor receptor-2, known as triple-negative breast cancers (TNBC), about 80% have a basal-like profile, in terms of both gene expression profile (3) and the display of established basal histology markers, such as cytokeratin 5/6 and the epidermal growth factor receptor (EGFR) (4).