Trastuzumab, a monoclonal antibody directed against the extracellular domain of HER2, which consists of four domains (domain I, II, III and IV) [9], is currently the first choice of treatment for HER2-positive breast cancer patients, as it improves overall survival and reduces the risk of disease recurrence when administered in combination with chemotherapy (for review see [10]). This evidence concerns the gene ERBB2 and breast cancer.