Checkpoint inhibitors targeting CTLA-4, PD-1, and PD-L1 now form part of first or second-line standard-of-care in melanoma, non-small cell lung cancer (NSCLC), advanced head and neck squamous cell cancer (HNSCC), renal cancer and urothelial cancer, among others (targets summarised in Table 1). This evidence concerns the gene PDCD1 and melanoma.