HER2is a target for ADCs, with trastuzumab emtansine (T-DM1) availablefor clinical use in patients with HER2 overexpressing breast cancer,and trastuzumab deruxtecan for patients with HER2 expressing breastcancer, gastric cancer, and gastroesophageal adenocarcinoma.5 HER3, however, has proven to be a more difficulttarget. This evidence concerns the gene ERBB2 and breast cancer.