In recent years, immune checkpoint inhibitors, such as programmed cell death protein-1 (PD-1), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), and T cell immunoglobulin and mucin domain-containing protein 3 (Tim-3), have attracted considerable attention as these molecules play critical roles in maintaining immune homeostasis, and their dysregulation is associated with many immune-related diseases, tumor development, chronic infections, and autoimmune diseases. The gene discussed is PDCD1; the disease is neoplasm.