Tumor cells exploit a repertoire of inhibitory ligands [e.g., PD-L1, B7 homolog 3 (B7-H3), and human leukocyte antigen (HLA)-E)] to engage checkpoint receptors [PD-1, Lymphocyte-activation gene 3 (LAG-3), Natural killer group 2 member A (NKG2A)] on T cells, thereby blunting TCR signaling and cytotoxicity activity. Here, PDCD1 is linked to neoplasm.