PDCD1 and breast cancer: PD1, also known as PDCD1 or CD279, is a membrane protein and expressed on T cells and pro-B cells.3–5 Its ligand, PD-L1, also referred to as B7-H1 or CD274, is a type 1 transmembrane protein that binds to PD1 and negatively regulates T cell function and survival.6 The expression of PD-L1 in the tumor microenvironment protects cancer from immune-mediated rejection.7 PD-L1 is up-regulated in a variety of human carcinomas, including breast cancer, ovarian cancer, colon cancer, melanoma and lung cancer.7,8 PD-L1 assists tumor cells evade the host immune system.