Several randomised controlled trials (RCTs) demonstrated that trastuzumab in addition to adjuvant chemotherapy improves disease-free survival (DFS) and overall survival (OS) in women with HER2-positive early breast cancer.4–11 The majority of patients in the landmark RCTs received 12 months of adjuvant trastuzumab.4,6,10 As subsequent studies investigating the efficacy of shorter durations of adjuvant trastuzumab are conflicting,12–20 12 months of trastuzumab remains the standard of care for HER2-positive early breast cancer. This evidence concerns the gene ERBB2 and breast carcinoma.