PDCD1 and neoplasm: To make matters worse, when TCLs arrive at tumor tissues to kill the tumor cells, immune-suppressive molecules on tumor cells (e.g., PD-L1 and PD-L2) and T cells (e.g., CTLA-4 and PD-1) suppress the activation of TCLs, enabling the tumor to evade the anti-cancer immune response and ultimately limiting the efficacy of the immunotherapy [2, 19, 27–30].