Tumour cells subvert immune surveillance by impairing neoantigen presentation, recruiting immune suppressor cells and expressing inhibitory molecules, thereby hindering the immune reaction through the blockade of co-stimulatory signals and activation of the immune checkpoint pathway (e.g., anti-PD-1 and its ligand PD-L1), leading to T cell anergy and exhaustion [22]. Here, PDCD1 is linked to neoplasm.