PD-L1, or “programmed-death ligand 1” (also known as CD274 or B7-H1) is a membrane bound glycoprotein in the B7 family of cell surface ligands involved in regulation of the immune system. PD-L1 is expressed on a variety of inflammatory-activated cells, some carcinomas, and in melanoma (ovary, colon, lung, breast, and renal cell carcinomas). PD-L1 expression on tumor cells is correlated with poor prognosis in patients with cancers such as NSCLC, esophageal cancer, and pancreatic carcinoma. Levels of PD-L1 are increased in the plasma of cancer patients as well as in cerebrospinal fluid of gliomas. sPD-L1 is a biomarker of poor survival in patients with B cell lymphoma, renal cell carcinoma, metastatic melanoma or lung cancer, and is associated with advanced tumor stage. PD-L1 contributes to immune evasion by binding to PD-1 and CD80 to suppress the activation and proliferation of T cells and induce apoptosis of activated T cells. Blocking the PD-1/PD-L1 pathway to prevent this immune evasion and restore anti-tumor immunity has emerged as a promising anti-cancer strategy.