This study demonstrated that the DFS benefit conferred by AC-T without trastuzumab in HER2-positive breast cancer patients is actually restricted to TOP2A co-amplified malignancies, which constituted a subset (35%) of the HER2-positive cancers, and is virtually indistinguishable from the benefit achieved by the addition of trastuzumab. The gene discussed is ERBB2; the disease is breast cancer.