PDCD1 and neoplasm: Constant exposure to a tumor antigen leads to the enhanced expression of inhibitory receptors such as PD-1, cytotoxic T lymphocyte antigen-4 (CTLA-4), T-cell immunoglobulin domain and mucin domain protein-3 (TIM-3), lymphocyte activation gene-3 (LAG-3), T-cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif domain (TIGIT), and band T lymphocyte attenuator (BTLA) [40,41].