T-Dxd was approved by the FDA in 2019 and by the EMA in 2021 for the treatment of unresectable or metastatic HER2+ breast cancer (after patients have received two or more anti-HER2 therapies), non-small cell lung cancer, and for locally advanced or metastatic HER2+ gastric or gastroesophageal junction adenocarcinoma, after a trastuzumab-based therapy. The gene discussed is ERBB2; the disease is gastroesophageal junction adenocarcinoma.