Endocrine therapies, including selective estrogen receptor modulators (SERMs), aromatase inhibitors (AIs), and selective estrogen receptor downregulators (SERDs), are the mainstay of treatments for estrogen receptor‐alpha (ERα)‐positive (ER+) breast cancer, which accounts for approximately 70% of all breast cancers (Musgrove & Sutherland, 2009). This evidence concerns the gene ESR1 and breast cancer.