Breast cancers are highly heterogenous and may display different characteristics of hormone receptors (HR) (estrogen receptor (ER) and progesterone receptor (PR)) and human epidermal growth factor receptor 2 (HER2) status, and together this information helps to distinguish different types of breast cancers: luminal A (HR+/HER2−, tumor grade 1 or 2), luminal B/HER2− (HR+/HER2−, tumor grade 3 or 4), luminal B/HER2+ (HR+/HER2+), triple negative (HR−/HER2−), or HER2 overexpressing (HR−/HER2+) [6]. This evidence concerns the gene ESR1 and breast cancer.