ESR1 and breast cancer: Based on these biomarkers, breast cancers are typically classified into four subtypes: luminal A (ER+, PR+, and HER2–), luminal B (ER+, PR+, and HER2+), HER2-enriched (ER–, PR–, and HER2+), and triple-negative breast cancers (TNBC) (ER–, PR–, and HER2–) [25–28].