ESR1 and breast cancer: BC is considered a heterogeneous disease, and the expression and amplification of human epidermal growth factor receptor type 2 (HER2), and the absence or presence of estrogen receptor (ER) and progesterone receptor (PR), cause this disease to be divided into three clinical subgroups; HER2-positive (HER2+), triple-negative (TN; ER–, PR–and HER2–), hormone-receptor (HR)-positive (HR+; ER+, PR+/– and HER2–) [97].