PDCD1 and neoplasm: Some co-stimulatory molecules such as programmed cell death protein-1 (PD-1), cytotoxic T-lymphocyte-associated protein (CTLA-4), T-cell immunoglobulin, mucin-domain containing-3 (TIM-3), and factor associated suicide (Fas; CD95) are intimately involved in tumor-driven immune escape mechanisms, due to their moderating effect on the activation, proliferation, and differentiation of T cells.