PD-1 also interacts with programmed death ligand-1 (PD-L1) and programmed death ligand-2 (PD-L2), which are mainly expressed on the surface of tumor cells or in the tumor matrix [61, 62]; these ligands activate PD-1, which then inhibits the proliferation of T cells and promotes the immune escape of tumor cells, playing an important role in immune suppression and cancer progression [63–65]. Here, CD274 is linked to cancer.