Breast cancer is a heterogeneous disease, the clinical classification sorts breast cancer into four major groups: Luminal A (ER+, [PR]-positive, human epidermal growth factor receptor 2 [HER2]-negative, low ki-67), Luminal B (ER+ and PR+/-, HER2+/-, or high ki-67), HER2-positive (HER2+ and ER-) and triple-negative (ER-/PR-/HER2-) based on immunohistochemical. This evidence concerns the gene ESR1 and breast cancer.