However, only 70% of breast cancers express the estrogen receptor (ER), and therefore, chemotherapy and therapies targeting genes with oncogenic alterations and related signaling pathways such as human epidermal growth factor receptor 2 (HER2), epidermal growth factor receptor (EGFR), BRAF, MAP2K1, ALK, CXCR4, etc., have been developed, based on advances in the understanding of molecular cancer biology. The gene discussed is ESR1; the disease is breast cancer.