PDCD1 and cancer: Checkpoint protein inhibitor antibodies (CPI), including cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) inhibitors (ipilimumab, tremelimumab) and the programmed cell death protein 1 pathway/programmed cell death protein 1 ligand (PD-1/PDL-1) inhibitors (pembrolizumab, nivolumab, durvalumab, atezolizumab), have entered routine practice for the treatment of many cancers.