ERBB2 and breast cancer: In a Phase 2 study, the addition of the PD-L1 inhibitor atezolizumab to T-DM1 induced a numerical increase in median PFS in previously treated PD-L1-positive HER2-positive advanced breast cancer patients,49 while a Phase 1b study to evaluate the safety, tolerability and activity of the PD-1 inhibitor pembrolizumab with T-DM1 in HER2-positive metastatic breast cancer is underway (Trial ID: NCT03032107).