ESR1 and cancer: Breast cancer (BC) is the most prevalent malignant tumor in women globally and one of the major causes of cancer‐related death in women.[1] Breast cancer is divided into four subtypes based on IHC results for estrogen receptor (ERα), progesterone receptor (PR), human epidermal growth factor receptor‐2 (HER2), and the tumor cell proliferation index (Ki‐67):[2] Luminal A, Luminal B, HER2 positive and basal‐like breast cancer (BLBC).