Immunotherapies have emerged as highly promising new approaches to combat cancer; these include antibodies against programmed cell death protein 1 (PD-1), programmed death ligand 1 (PD-L1), and CTLA4 (checkpoint blockade), and modified immune cells that target tumors, exemplified by chimeric antigen receptor T (CAR T) cells (1, 2). Here, PDCD1 is linked to cancer.