PDCD1 and cancer: The strategy of blocking adaptive immune checkpoints including programmed cell death protein 1 (PD-1), programmed death-ligand 1 (PD-L1), and cytotoxic T-lymphocyte associated protein 4 (CTLA-4) offers curative immunotherapy for some patients with otherwise terminal cancer diagnoses — however, only a minority of patients respond to immune checkpoint blockade (ICB) therapy and many cancer types remain inaccessible by this treatment (1, , , , –6).