CD274 and cancer: Recently, programmed death-ligand 1 (PD-L1), the ligand for the cytotoxic T lymphocyte immune checkpoint receptor programmed death 1 (PD-1) during immunosuppression processes within primary tumor tissues, has also been detected on the cell surfaces of CTCs derived from cancer patients [296,297], implying that T cell-mediated adaptive immunosurveillance is also in effect in the circulation and CTCs may escape such immune assault by the engagement of PD-1/PD-L1 immune checkpoint [36].