ESR1 and neoplasm: Increased PD-L1 expression has been reported in multiple studies of MBC, with one study of 75 MBCs reporting PD-L1 overexpression in 46% of cases, with overexpression defined as 2+ staining in >5% of tumor cells, compared to just 9% in TNBC and 6% in HER2+ or ER/PR+ tumors (8).