The combination of conventional chemotherapy with trastuzumab (or Herceptin)—a humanized monoclonal antibody that binds to domain IV of the extracellular segment of HER2—has proven successful in improving the overall survival of metastatic BC patients; nevertheless, about 50–60% of them do not respond to them, presenting with a lower clinical benefit and lower index of disease-free survival due to mechanisms of drug resistance [66]. The gene discussed is ERBB2; the disease is breast cancer.