ESR1 and breast carcinoma: Many researchers have classified breast cancer cells into four types according to the type of hormone receptor: HER2 type (ER+, PR−, HER2+), luminal A (ER+, PR+, HER2−), luminal B (ER+, PR−, HER2+), and triple-negative breast cancer (ER−, PR−, HER2−) [4].