Breast cancer is defined into 3 clinically relevant subtypes according to estrogen receptor (ER) expression, progesterone receptor (PR) expression and human epidermal growth factor 2 (ERBB2; formerly HER2) gene amplification: ER-positive/PR-positive, HER2/ERBB2-positive, and triple-negative (lacking expression of all three molecular markers) (2). The gene discussed is ESR1; the disease is breast cancer.