T-DXd has demonstrated clinically meaningful activity across a broad range of HER2-expressing solid tumors (2) and is currently approved as a standard of care treatment for HER2-expressing metastatic breast cancer, HER2-positive gastric or gastroesophageal junction adenocarcinoma, and HER2-mutant non-small cell lung cancer (3–8). The gene discussed is ERBB2; the disease is gastroesophageal junction adenocarcinoma.