Based on the presence or absence of molecular markers for the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor 2 (ERBB2; formerly HER2), breast cancer is categorized into three major subtypes, including hormone receptor–positive/ERBB2-negative, ERBB2-positive, and triple-negative. The gene discussed is ERBB2; the disease is breast carcinoma.