CD274 and breast cancer: Thus, testing of PD-L1 in breast cancer tissue currently lacks standardization to encompass the diversity in the assays (IHC, gene expression), antibodies for testing, assessment systems and thresholds for classifying the PD-L1 expression (positive or negative), tumor microenvironment compartments included analysis (tumor cells, immune cells, or both) and nature of the tumor samples (primary, metastatic), along with a lack of appropriate widespread antibody platforms for testing PD-L1.