ESR1 and breast cancer: Breast cancer is broadly classified based on a hormonal status analysis using immunohistochemistry as luminal A [progesterone receptor (PR)–positive, estrogen receptor (ER)–positive, and human epidermal growth factor 2 (Her2)–negative] and luminal B (ER-positive, PR-positive/negative, and Her2-positive) being the estrogen-positive subtypes, Her2 enriched, and triple-negative breast cancer (Perue et al., 2000; Yeh and Mies, 2008; Penault-Llorca and Viale, 2012).