Because of the crucial roles of progesterone receptor (PR), estrogen receptor (ER) and human epidermal growth factor receptor 2 (HER2) in breast cancer carcinogenesis, it is classified based on their expression: (1) ER+, estrogen receptor positive; (2) HER2+, overexpressing human epidermal growth factor receptor 2, which can be ER+ or ER-; (3) TNBCs, triple negative subtype that do not express estrogen, progesterone, and HER2 receptors. The gene discussed is ESR1; the disease is breast cancer.