ESR1 and breast cancer: Breast cancer can be classified into four different molecular subtypes: luminal A (estrogen receptor/progesterone receptor-positive [ER/PR+], human epidermal growth factor receptor 2-negative [HER2-]), luminal B (ER/PR+, HER2+), triple negative (ER/PR-, HER2-), and HER2-enriched (ER/PR-, HER2+) 7, of which the ER+ subtype is the most prevalent.