Based on the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal receptor 2 (HER2), breast cancer is subdivided into the following groups: luminal A (ER+/PR+/HER2-), luminal B (ER+/PR+/HER2+), HER2 enriched (ER-/PR-/HER2+), and basal-like, including triple-negative breast cancer (TNBC), which lacks ER, PR, and HER2. This evidence concerns the gene ESR1 and breast cancer.