Based on the expression levels of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) (Brenton et al., 2005), breast cancer is classified into three molecular subgroups, ER+, HER2+, and triple-negative breast cancer (TNBC). The gene discussed is ERBB2; the disease is triple-negative breast carcinoma.