Although immune checkpoint inhibitors (ICIs) targeting cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programmed cell death-1 (PD-1), and programmed death-ligand 1 (PD-L1) have achieved durable responses in several cancers, their therapeutic benefit remains limited to a subset of patients, largely due to immune evasion, tumor heterogeneity, and immunosuppressive features of the tumor microenvironment (TME). The gene discussed is PDCD1; the disease is cancer.