PDCD1 and cancer: To combat the immune attack and maintain the balance with the host, cancer cells develop a series of approaches such as (1) downregulation of antigen presentation to make them invisible to immunity/the immune system, (2) production of immunomodulatory cytokines to establish an immune suppressive environment, and (3) expression of surface immune checkpoint proteins such as programmed cell death 1 ligand 1 (PD-L1) that engages with its receptor, programmed cell death protein 1 (PD-1), on immune cells to inhibit the T-cell-mediated immune response.