PDCD1 and cancer: Notably, they are expressed on malignant tumors with epigenetic changes and on immune-related cells, such as programmed death ligand 1 (PD-L1) expressed on tumor cells and CTL-associated protein 4 (CTLA-4) and programmed cell death 1 (PD-1) expressed on T cells, which results in tumor immune escape, tumor progression, and metastasis [141–143].