ESR1 and breast cancer: To date, three main drug types are used for hormone-dependent BC treatment: SERMs, such as tamoxifen, which antagonizes the ER at the nuclear level [19]; selective ER downregulators (SERDs), such as fulvestrant, which induce destabilization and degradation of ER; and AIs, which reduce estrogen production in the peripheral tissues and within the tumors through inhibition of the aromatase enzyme [20].