ERBB2 and breast cancer: On the basis of the expression of tissue-based biomarkers, such as the hormone receptor (HR) and the epidermal growth factor 2 (ERBB2), breast cancer is classified into the following five main subtypes: luminal A (HR+/ERBB2−), luminal B (HR+/ERBB2+), normal like (HR+/ERBB2−), ERBB2+, and triple negative (TNBC) (HR−/ERBB2−), associated with different prognosis and outcome [2].