ESR1 and neoplasm: Endocrine therapies, including aromatase inhibitors (AIs, e.g., letrozole), selective ER modulators (SERMs, e.g., tamoxifen), and selective ER degraders (SERDs, e.g., fulvestrant) that block ER signaling via ER inhibition, modulation, or degradation, and hence estrogen-promoted tumor growth, are mainstays of treatment for patients with HR+/HER2− breast cancer4.