ER is a well-established predictive marker in BC used for consideration of endocrine therapy (Rakha et al. 2022) that for ER-positive patients include aromatase inhibitors (AI), which inhibit the estrogens synthesis and anti-estrogens: selective ER modulators (SERMs; e.g., tamoxifen) and selective ER downregulators (SERDs; e.g., fulvestrant), which interfere with estrogen-dependent pathways (Nicolini et al. 2018; Almeida 2020; Awan and Esfahani 2018; Hanker et al. 2020). The gene discussed is ESR1; the disease is breast cancer.