PDCD1 and neoplasm: Excessive PD-1 expression on tumor-infiltrating leukocytes (TILs) is often coexpressed with other inhibitory receptors such as T cell immunoglobulin and mucin domain–containing protein 3 (Tim-3), lymphocyte-activation gene 3 (Lag-3), and cytotoxic T lymphocyte–associated protein 4 (CTLA-4) and drives T cell exhaustion (Tex), especially terminal exhaustion (4, 5).