ERBB2 and breast cancer: The lines of evidence that prompted consideration of HER2 as a therapeutic target were the following: transfection of the HER2 gene induces the malignant phenotype; HER2 is overexpressed in 17–20% of human breast cancers; the main cause of HER2 overexpression is gene amplification; HER2 overexpression or gene amplification carries a poor prognosis in patients with breast cancer [5]; and finally, at the end of the 1990s it was shown that monoclonal antibodies directed against HER2 were capable of exerting an antitumor effect.