The activation status of HER2, and not just its overexpression, is a crucial event determining both the aggressive biological behavior of HER2 positive-breast carcinomas and the responses of HER2 positive-breast tumors to chemotherapy, anti-estrogens and, more importantly for this study, to the anti-HER2 antibody trastuzumab [41-45]. This evidence concerns the gene ERBB2 and breast carcinoma.