Pembrolizumab, an immune checkpoint inhibitor (ICI) that exhibits antitumour effects by inhibiting programmed cell death 1 (PD-1), has been approved for the treatment of malignant melanoma, non-small-cell lung cancer (NSCLC), urothelial carcinoma, renal cell carcinoma, and other types of cancer. This evidence concerns the gene PDCD1 and cancer.