The classification of breast tumors involves the molecular expression of specific genes using the PAM50 classification, which categorizes them into five clinicopathological surrogates: luminal A (LumA), luminal B (LumB), HER2-overexpressing (HER2 +), basal epithelial-like (BL), and normal-like (NL)1. The gene discussed is ERBB2; the disease is breast neoplasm.