Page DB, et al. - This work aimed to retrospectively identify HER2-amplified, stage I–III breast cancers with monosomy of chromosome 17 (m17) and characterize the impact of trastuzumab treatment. In this study, patients with HER2-amplified m17 cancers treated with trastuzumab indicated outcomes comparable to patients from the large phase III adjuvant trastuzumab trials who were HER2-positive. Findings thereby supported the critical role of HER2-directed therapy in this patient population.