Breast cancer can be divided into four molecular subtypes: (1) Luminal A: Estrogen receptor (ER+), progesterone receptor (PR+), human epidermal growth factor receptor 2 (HER-2+) (and Ki67 < 14%]; (2) Luminal B (ER+, PR+, HER2+, and Ki67 > 14% or ER+, PR+, and HER2+); (3) HER-2 positive (ER-, PR-, and HER-2+), which represents 25% to 30% of breast cancers; and (4) the Triple-Negative Breast Cancer (TNBC: ER-, PR-, and HER-2-), accounting for 12% to 24% of all breast cancers. The gene discussed is ESR1; the disease is breast cancer.