Because immune checkpoint inhibitors (ICIs) are primarily effective in early-stage tumors [207], the ASTEFANIA clinical trial (NCT04873362) is investigating the combination therapy of PD-1/PD-L1 inhibitor (atezolizuman) plus T-DM1 over T-DM1 plus placebo in early-stage HER2+ BC and residual disease after neoadjuvant HER2-targeted therapy plus chemotherapy. Here, ERBB2 is linked to breast cancer.