PD-L1 (programmed cell death ligand 1, also known as CD274 or B7-H1) is a 290 aa type I transmembrane protein. PD-L1 is expressed constitutively on murine T cells, B cells, DCs, macrophages, mesenchymal stem cells and cultured bone marrow-derived mast cells. In addition, PD-L1 is also expressed on many nonhematopoietic cell types, including vascular endothelial cells, epithelial cells, muscle cells, hepatocytes, pancreatic islet cells, astrocytes in the brain, placental syncytiotrophoblasts, and cells in cornea, iris-ciliary body and retina of eye. PD-L1 is frequently upregulated in a wide variety of solid tumors, including melanoma, ovarian, lung, glioblastoma, breast, and pancreatic cancers. PD-L1 and PD-L2 are two ligands of PD-1. Engagement of PD-1 by PD-L1 or PD-L2 transduces a signal that inhibits T-cell proliferation, cytokine production, and cytolytic function. It is critical for the regulation of T cell function during tolerance, autoimmunity and infection.