Categorization into four principal subtypes (luminal A, luminal B, human epidermal growth factor receptor 2-positive [HER2+], and triple-negative breast cancer [TNBC]) is facilitated by disparities in estrogen receptor (ER), progesterone receptor (PgR), HER2, and the Ki67 proliferation index. This evidence concerns the gene ERBB2 and triple-negative breast carcinoma.