DS-8201a was tested in a phase I dose-escalation trial with multiple expansion cohorts: HER2-positive, trastuzumab-DM1 (T‐DM1) pretreated MBC patients; patients with HER2-positive gastric cancer; MBC patients with low HER2-expression (HER2 1+/2+, HER2/neu ISH negative); and a cohort of patients with other HER2-positive or HER2/neu mutated solid tumours. This evidence concerns the gene ERBB2 and gastric cancer.