PDCD1 and melanoma: Inhibition of the PD-1/PD-L1 axis contributes to important clinical advances in cancer therapy, including melanoma (1, 2), non-small cell lung cancer (NSCLC) (3–5), renal cell carcinoma (RCC) (6), Hodgkin's lymphoma (7, 8), bladder cancer (9, 10), head and neck squamous cell carcinoma (HNSCC) (11–13), Merkel-cell carcinoma (14), urothelial carcinoma (15), and microsatellite instable-high (MSI-H) or mismatch repair-deficient (dMMR) solid tumors (16–18).