The primary mechanism of HER2 activation in breast cancer is its gene amplification on the long arm of chromosome 17 (17q12-21-21.32) [34]; this consequently leads to the overexpression of HER2 protein (receptor) (Figure 1b) causing homo- or heterodimerization with other HER family members and resulting in auto- and transphosphorylation, which in turn activates several signaling pathways [19]. This evidence concerns the gene ERBB2 and breast cancer.