These are generally defined as: basal-like breast cancer, which generally corresponds to estrogen receptor (ER) negative, progesterone receptor (PR) and HER-2 negative (i.e., triple negative), luminal A (ER positive, low grade), luminal B (ER positive, high grade) and HER-2 (or ErbB2) positive [137]. This evidence concerns the gene ESR1 and breast cancer.