BC can be classified as HER2-rich type (hormone receptor (HR), estrogen receptor (ER), progesterone receptor (PR)−/HER2+), basal-like type (triple-negative breast cancer, TNBC, ER/PR/HER2−), normal mammary type [3,4], and luminal type, including luminal A (HR+/HER2−) and luminal B (triple positive) [4,5]. This evidence concerns the gene ESR1 and breast cancer.