ERBB2 and breast cancer: Breast cancer subtypes are classified as luminal A (Estrogen Receptor Alpha (ERα) positive, Progesterone Receptor (PR) positive, Erb-B2 receptor tyrosine kinase 2 (HER2) negative), luminal B (ERα+/PR+/HER2+), HER2+, or basal (ERα-/PR-/HER2-; triple negative BCa) [16].