ESR1 and cancer: Based on the hormone receptor (estrogen receptor [ER] or progesterone receptor [PR]) and human epidermal growth factor receptor 2 (HER2) status, breast cancer is classified into four main subtypes; hormone receptor-positive/HER2-negative (or luminal A), hormone receptor-positive/HER2-positive (or luminal B), hormone receptor-negative/HER2-positive (HER2-positive or HER2-enriched), and triple-negative breast cancer (TNBC), and the choice of systemic treatment depend on the cancer subtypes (2, 3).