ESR1 and breast cancer: Breast cancer is clinically categorized into three major subtypes, which show distinct characteristics and reflect patient prognosis: hormone receptor (HR)-positive type (oestrogen receptor [ER]+, progesterone receptor [PgR]+/−, and HER2-), HER2-positive type (ER-, PgR +/−, and HER2+), and triple-negative (TN) type (ER-, PgR-, and HER2-) [94].