Breast cancer is a heterogeneous disease classified into clinical subgroups based on the assessment of three main treatment predictive biomarkers: protein overexpression of the estrogen receptor (ER) and progesterone (PR) receptor, and overexpression/gene amplification of the human epidermal growth factor receptor 2 (ERBB2/HER2) gene. The gene discussed is ERBB2; the disease is breast carcinoma.