ERBB2 and breast carcinoma: A high degree of breast cancer heterogeneity led to its classification into four distinct molecular subtypes, which can vary in tumor genomics and the cell type from which tumor initiation occurs: (1) luminal A, (2) luminal B, (3) human epidermal growth factor receptor 2 (HER2)-positive, and (4) triple-negative breast cancer (TNBC).