PDCD1 and neoplasm: In tumor microenvironment (TME) T cells, the expressions of costimulatory signal molecules including CD137, CD28, and OX40 are often significantly decreased [24–26], while the expressions of costimulatory signal molecules such as cytotoxic T-lymphocyte-associated protein 4 (CTLA4), programmed cell death protein 1 (PD-1), and T-cell immunoglobulin mucin-3 (TIM-3) are significantly increased [27–29].