PDCD1 (programmed cell death 1) encodes an inhibitory immune checkpoint receptor that negatively regulates T cell-mediated anti-tumor immunity. PDCD1 interacts with its ligand CD274/PDCD1L1 to inhibit cytotoxic T lymphocyte effector function, thereby suppressing anti-tumor immune responses 1. Tumors exploit this PDCD1-mediated inhibitory pathway to evade immune destruction and facilitate survival 1. Blockade of PDCD1 signaling reverses T cell exhaustion and restores anti-tumor immunity, providing rationale for PD-1 checkpoint immunotherapy 234. Beyond cancer, PDCD1 polymorphisms are associated with multiple autoimmune conditions. Genetic variants rs2227982 and rs2227981 increase type 1 diabetes risk in East Asians 5, while rs10204525 associates with polyarticular juvenile idiopathic arthritis 6. In ankylosing spondylitis, PDCD1 promoter hypermethylation correlates with reduced gene expression and disease severity markers 7. PDCD1 polymorphisms also influence cutaneous melanoma risk and prognosis through altered T lymphocyte regulation 8. Pan-cancer analysis demonstrates PDCD1 expression correlates with tumor mutational burden, microsatellite instability, and immune cell infiltration, serving as a prognostic marker across multiple cancer types 9. These findings highlight PDCD1's critical dual role in cancer immunity and autoimmune disease pathogenesis.