ERBB2 and breast cancer: Breast cancer has five molecular subtypes depending on differential gene expression: Luminal A (hormone receptor+ and HER2−), luminal B (hormone receptor+ and HER2−/+), basal-like or triple-negative breast cancer (TNBC) (hormone receptor− and HER2−), HER2 enriched (hormone receptor− and HER2−/+), normal-like (hormone receptor+ and HER2−) [5] (Figure 1).