ESR1 and cancer: Breast cancer (BC) is one of the most common malignancies and a leading cause of cancer-related mortality among women worldwide.[1,2] BC can be divided into 4 molecular sub-types[3]: Luminal A, Luminal B, HER-2 positive, and triple-negative, based on the expressions of the hormone receptors (estrogen receptor [ER] and progesterone receptor [PR]), human epidermal growth factor receptor-2 (HER-2), and ki-67 of BC tissues.