For example, the programmed cell death protein 1 (PD1) receptor, which can be expressed on the surface of tumor-infiltrating lymphocytes, binds to PD1 ligands on the surface of tumor cells (including PD-L1 and PD-L2, usually PD-L1), transmits negative regulatory signals, and causes apoptosis or loss of function of effector T cells that exert anti-tumor effects 76-78. Here, PDCD1 is linked to neoplasm.