In the breast cancer chapter, levels of estrogen receptor (ER) and progesterone receptor (PR) expression, human epidermal growth factor receptor 2 (HER2) or erythroblastic oncogene B (ERBB2) expression, histologic grade, regional lymph node involvement, distant metastases, and prognostic biomarkers are included in order to confer precise prognosis and guide treatment decisions (14, 16). This evidence concerns the gene ESR1 and breast cancer.