This study will evaluate the efficacy, safety and patient-reported outcomes of trastuzumab
emtansine plus atezolizumab compared with trastuzumab emtansine plus placebo in participants
with HER2-positive and PD-L1-positive LABC or MBC.Participants must have progressed either
during or after prior trastuzumab- (+/- pertuzumab) and taxane-based therapy for LABC/MBC; or
during (or within 6 months after completing) trastuzumab- (+/-pertuzumab) and taxane-based
therapy in the neoadjuvant and/or adjuvant setting.