ERBB2 and breast cancer: At the molecular level, four subgroups of breast cancers exist based on estrogen (ER) and progesterone [5] gene heterogeneity [5]: luminal A (ER or PR+, Human Epidermal Growth Factor Receptor 2- (HER2-)), luminal B (ER- or PR+, HER2+), ER-PR-HER2+, and triple-negative (ER-PR-HER2-) types.