ERBB2 and breast cancer: Overexpression of HER2 has an important pathogenic role in breast tumors as evidenced by the fact that it promotes malignant behavior in human mammary epithelial cell lines, that it is sufficient to cause invasive mammary cancers in MMTV-Neu transgenic mice and that targeted therapy against HER2 is effective in patients with HER2-positive tumors [1–5].