ESR1 and breast cancer: Based on molecular and pathological determinations, breast cancer is divided into three main subclasses with differing clinical management and prognostic assessments: the estrogen receptor (ER) and progesterone receptor (PR)-positive tumors (called luminal subtypes), the human epidermal growth factor receptor 2 (HER2 or ERBB2) enriched tumors (called HER2), and the ER-negative, PR-negative, and HER2-negative tumors, also called triple-negative breast cancers (TNBCs)3–6.