ESR1 and breast cancer: It is highly efficacious in the adjuvant setting for early stage, estrogen receptor-positive breast cancer as well as in the chemoprevention of breast cancer for patients at high risk.1–3 Yet, a significant number of patients with breast cancer as well as the healthy high-risk population choose to forego these benefits, often based on concern over the risk of endometrial cancer, a known but uncommon side effect of tamoxifen.