PDCD1 and cancer: Immune checkpoint inhibitors (ICIs) including programmed cell death 1 (PD-1) inhibitors, programmed cell death 1 ligand 1(PD-L1)inhibitors and cytotoxic T- lymphocyte-associated antigen 4 (CTLA-4) inhibitors have revolutionized cancer therapy and become the standard treatment for a number of malignancies in the past few years (1, 2).