PDCD1 and neoplasm: Immune checkpoint molecules, such as programmed cell death protein 1 (PD-1), programmed death-ligand 1 (PD-L1), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), and T-cell immunoglobulin and mucin domain-containing protein 3 (TIM-3), play pivotal roles in regulating anti-tumor immune responses [13, 14].