PDCD1 and cancer: Immune checkpoint therapy (ICT) targeting proteins such as cytotoxic T lymphocyte–associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1), or programmed death-ligand 1 (PD-L1) has been transformative in many cancer types; providing durable clinical responses and markedly improved overall survival in a subset of patients (1–3).