CD274 and neoplasm: When the tumor occurs, a high-level expression of PD-1 in the infiltrating T cells is induced through the microenvironment, whereas a corresponding high expression of PD-L1/PD-L2 in tumor cells (TCs), in turn, leads to the constitutive activation of PD-1/PD-L1 and PD-1/PD-L2 signaling; then, T-cell function is attenuated or even inhibited, which prevents them from signaling to attack the TCs, potentially allowing TCs to evade immune surveillance and to further progress (Figure 1).