For prognostic assessment and clinical decision-making, breast cancer has been divided into three major subtypes based on the presence or absence of molecular markers for estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (ERBB2, Erb-B2 Receptor Tyrosine Kinase 2, also known as HER2). The gene discussed is ERBB2; the disease is breast cancer.