One of the most well-characterized immune checkpoints, programmed death-1 (PD-1), is expressed on activated T cells, macrophages, regulatory T cells (Tregs), and natural killer (NK) cells, while its ligands PD-L1 and PD-L2 are expressed on T cells, dendritic cells (DCs), and a variety of other cell types, including tumor cells [22,24]. The gene discussed is CD274; the disease is neoplasm.