PDCD1 and neoplasm: The best characterized inhibitory receptors on tumor-infiltrating lymphocytes (TILs) are programmed cell death protein 1 (PD-1), cytotoxic T lymphocyte associated-antigen 4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3) and T cell immunoglobulin and mucin-domain containing 3 (TIM-3) [14,15,16,17].