PD1 or CD279 is another major receptor in immune checkpoints that is expressed on tumor-infiltrating cytotoxic T lymphocytes (CTLs).66, 67 Engagement of PD1 by its ligand (mostly PD-L1) triggers the phosphorylation of the cytoplasmic tail of PD1, and consequently, the inhibitory signal takes effect in CTLs.68 Here, PDCD1 is linked to neoplasm.