ESR1 and breast cancer: Breast cancer is a heterogeneous disease; its subtypes have been classified as triple negative (TN) (ER-negative [ER-], PR-, and HER2-), luminal A (ER-positive [ER+] or PR+ plus HER2-), luminal B (ER+ or PR+ plus HER2+), and HER2-enriched (ER-/PR-/HER2+) subtype [8-16].