A phase I trial tested trastuzumab deruxtecan (T-DXd), a humanised anti-HER2 antibody conjugated to a potent topoisomerase I inhibitor, in patients with advanced ERBB2-expressing or ERBB2-mutant solid tumours, demonstrating promising antitumour activity with an acceptable safety profile in patients with ERBB2-amplified breast or gastric cancer. The gene discussed is ERBB2; the disease is gastric cancer.