CD274 and cancer: The principal ligand of PD-1, programmed death-ligand 1 (PD-L1), also known as B7-H1 or CD274 (Ishida et al., 1992), is frequently expressed within the tumor microenvironment, including in cancer cells, antigen presenting cells (APCs), tumor-infiltrating macrophages, T cells, B cells, dendritic cells, and mesenchymal stem cells (Weber, 2010; Pardoll, 2012).