Programmed death-ligand 1 (PD-L1) is a 40kDa type 1 transmembrane protein that suppresses immune system. PD-L1 is expressed in hematopoietic and nonhematopoietic cells including T cells and B cells and various types of tumor cells. PD-L1 binds to its receptor Programmed death 1 (PD1). During infection or inflammation, PD1PD-L1 interaction is important for preventing autoimmunity. In tumor microenvironments, PD1-PD-L1 interaction provides an immune escape for tumor cells through cytotoxic T-cell inactivation. Expression of this gene in tumor cells is considered to be prognostic in many types of human cancers. In recent years, PDL1 has become an important biomarker and immunotherapy target for many types of malignancies.