In this open-label, two-arm phase Ib/II study, we evaluated the anti-tumor effect of the anti-PD-L1 monoclonal antibody atezolizumab in combination with either the MEK inhibitor cobimetinib in patients with TP53-mutated ER + MBC (arm COBI) or with the MDM2 antagonist idasanutlin in patients with TP53-wild-type ER + MBC (arm IDA). This evidence concerns the gene MDM2 and neoplasm.