PDCD1 and neoplasm: A group of inhibitory or stimulatory molecules expressed on immune cells, antigen-presenting cells (APCs), tumor cells, or other types of cells are regarded as immune checkpoints, including programmed cell death-1 (PD-1), cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), cluster of differentiation 28 (CD28), cluster of differentiation 80 (CD80), galectin-9 (Gal-9), and T cell immunoglobulin and mucin domain 3 (TIM-3) (4–7).