Immune therapy with antibodies to programmed cell death-1 (PD-1)/programmed death-ligand 1 (PD-L1) (PD-1 blockade) has been approved for treatment of patients with a growing list of cancers including malignant metastatic melanoma, non-small cell lung cancer, head and neck and esophageal squamous cell carcinoma, renal cell carcinoma, Hodgkin’s lymphoma, bladder cancer, cervical cancer, gastric cancer, hepatocellular carcinoma, Merkel cell carcinoma, and solid tumors with microsatellite instability. This evidence concerns the gene PDCD1 and Merkel cell skin cancer.