For two ADCs, trastuzumab-emtansine (T-DM1) and trastuzumab deruxtecan (T-DXd), patient selection based on traditional biomarkers such as expression of HER2 by immunohistochemistry (IHC) or in situ hybridization has played a role for HER2-positive breast cancer. The gene discussed is ERBB2; the disease is breast carcinoma.