In this multicenter, retrospective observational study conducted by the Kinki Breast Cancer Study Group-Translational Research (KBCSG-TR), we aimed to examine real-world effectiveness following T-DM1 discontinuation (post–T-DM1 treatment) in patients with HER2-positive, unresectable and/or metastatic breast cancer. The gene discussed is ERBB2; the disease is breast carcinoma.