HER2 gene amplification occurs in 20%‐25% of breast cancers and is associated with high relapse and poor prognosis rates.1, 2, 3 Trastuzumab (herceptin) is a monoclonal antibody that inhibits downstream signalling of intracellular transduction by targeting the extracellular domain of the HER2 gene. The gene discussed is ERBB2; the disease is breast carcinoma.