PDCD1 and neoplasm: The discovery of tumor escape mechanisms from immune surveillance-a theory presented in 1967 by Burnet and Thomas-opened a new era in the development of immunotherapies aimed at negative immune checkpoints, such as programmed death 1 (PD-1), programmed death ligand 1 (PD-L1), cytotoxic T lymphocyte antigen 4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3), T-cell immunoglobulin mucin-3 (TIM-3).