CD274 and neoplasm: Programmed death ligand 1 (PD-L1) is a major prognostic biomarker for immune therapy in many cancers, and it is widely expressed on various cell types, such as tumor cells, monocytes, macrophages, natural killer (NK) cells, dendritic cells (DCs), and activated T cells, as well as on immune-privileged sites such as the brain, cornea, and retina; it also plays a critical role in tumor progression and immunotherapy [176].