Monoclonal antibodies that target the immunological checkpoints PD-1 (programmed cell death protein 1; CD279) and PD-L1 (programmed death ligand 1; CD274) have proven successful in the treatment of multiple cancers, notably metastatic melanoma, where PD-1 blockade is now part of the standard of care1–4. This evidence concerns the gene PDCD1 and metastatic melanoma.