ESR1 and breast cancer: Accordingly, current pharmacological treatment for ER+ BC includes aromatase inhibitors (AIs) [8,9], selective ERα modulators (SERMs) (e.g., tamoxifen (TAM)), which are partial antiestrogens [5], or selective ERα degraders (SERDs) (e.g., fulvestrant), which are pure antiestrogens [10,11].