ESR1 and breast cancer: Based on this, breast cancer can be broadly grouped into five types, namely: i) Progesterone receptor (PR)-positive, estrogen receptor (ER)-positive and human epidermal growth factor 2 (Her2)-negative (luminal A); ii) ER-positive, PR-positive/negative and Her2-positive (luminal B); iii) Her2-overexpressing, ER- and PR-negative; iv) ER-, PR- and Her2-negative (basal-like or triple-negative), and v) normal-like (expression status similar to luminal A and resemble normal breast profile) (3–5).