ESR1 and breast cancer: Nevertheless, hormonal therapies targeting either estrogen production (i.e., aromatase inhibitors) or the hormone receptor itself such as selective ER modulators (SERMs; i.e., tamoxifen) and selective ER degraders (SERDs; i.e., fulvestrant) are widely used to block the mitogenic action of estrogens in patients with ER-positive breast cancer (Renoir, 2012; Farcas et al., 2021), contributing to the decline in mortality from breast cancer in recent decades (Iwase et al., 2021).