Although trastuzumab prolongs the survival of HER2-positive breast cancer patients and is the standard of care for HER2-positive breast cancer (NCCN Clinical Practice Guidelines in Oncology Breast Cancer (2012) V. 2), resistance to the drug is common, with the overall response rates (ORRs) with single-agent trastuzumab being in the range of 15–26% [9, 10]. The gene discussed is ERBB2; the disease is breast cancer.