ESR1 and breast cancer: Based on pathological and immunohistochemical markers, breast cancer can be classified into four subtypes: Luminal A (the most common molecular subtype: ER+, PR+, HER2-), Luminal B (ER+, PR+, HER2+), HER2-positive (ER-, PR-, HER2+), and triple-negative (ER-, PR-, HER2-).