Endocrine therapy, as the mainstay of ER+ breast cancer treatment, can disrupt the interaction of estrogen with the ER ligand binding domain and inhibit the ER signaling pathway either by selective ER modulators/degraders (SERMs/SERDs) antagonizing ER or by aromatase inhibitors (AIs), which decreases the level of systemic estrogen by blocking the conversion of androgens to estrogens. Here, ESR1 is linked to breast cancer.