The blocking of HER2 activity with trastuzumab, which binds to the extracellular domain IV of the HER2 receptor, prevents dimerization and inhibits downstream signaling, which results in cell cycle arrest and apoptosis and leads to improved outcomes for patients with HER2-positive disease.1 However, in women with metastatic breast cancers (MBCs), resistance usually occurs. This evidence concerns the gene ERBB2 and maternal uniparental disomy of chromosome 20.