PDCD1 and breast cancer: Accordingly, a monoclonal antibody which blocks PD-L1 binding to the PD-1 receptor on T cells was approved by the Food and Drug Administration to treat several tumor types and PD-L1 positive unresectable locally advanced and metastatic triple negative (TN) breast cancer [19], with many studies showing that PD-1 and PD-L1 are highly expressed in HER2-positive and TN cancer subtypes [20,21,22,23,24].