A clinical trial to compare trastuzumab emtansine plus atezolizumab with trastuzumab emtansine alone in patients with a particular type of breast cancer that has spread to other parts of the body and who already received trastuzumab- (with or without pertuzumab) and taxane-based therapy.

A Study of Trastuzumab Emtansine in Combination With Atezolizumab or Placebo as a Treatment for Participants With Human Epidermal Growth Factor 2 (HER2)-Positive and Programmed Death-ligand 1 (PD-L1)-Positive Locally Advanced (LABC) or Metastatic Breast Cancer (MBC)