Trastuzumab (Trast), also known as Herceptin, is a humanized monoclonal antibody targeting the extracellular domain of erythroblastic leukemia viral oncogene homolog-2 also known as HER2 (ErbB-2/HER-2), which is overexpressed in 25–30% of breast cancers and their metastases [7]. The gene discussed is ERBB2; the disease is breast cancer.