A definition of intrinsic subtypes of breast cancer was proposed: luminal A (ER+ and/or PR+, Ki67 low, and Her2−), luminal B (ER+ and/or PR+, Ki67 high, and/or Her2+), Her2-positive (ER-, PR-, and Her2+), and triple-negative (ER-, PR-, Her2-) [8,9]. This evidence concerns the gene ESR1 and breast cancer.