ESR1 and breast cancer: Waks et al. classified breast cancer into three major subtypes based on the presence or absence of molecular markers for estrogen receptor (ER) and progesterone receptor (PR) and human epidermal growth factor 2 (HER2), namely ER+/PR+/HER2- (luminal A), HER2+, and triple-negative breast cancer (TNBC), which have a negative indicator in all three standard molecular markers [9].