Despite this effectiveness, in early stage settings, over 20% of HER2-positive breast cancer patients are insensitive to trastuzumab treatment due to de novo resistance, and in metastatic settings most HER2-positive breast-cancer patients may develop resistance to trastuzumab after 6 to 8 months of exposure to the treatment and suffer disease progression within one year [4]. This evidence concerns the gene ERBB2 and breast carcinoma.