ESR1 and breast cancer: Approximately 60–70% of breast cancers may express ER and/or PR and are called luminal A subtype (ER+ and/or PR+, HER2−, Ki-67 low) and luminal B subtype (ER+ and/or PR+, HER2+ or HER2−but high Ki-67), approximately 20–30% overexpress HER2 (HER2 enriched), and approximately 10% do not express any of the three receptors and are classified as “triple negative” (basal-like tumors) [6,15].