PDCD1 and metastatic melanoma: The development of blocking antibodies that target inhibitory PD-1/PD-L1 or CTLA-4/CD80/CD86 pathways has led to significant improvements in the prognosis of patients suffering from various cancers including metastatic melanoma, non-small cell lung cancer (NSCLC), renal cell carcinoma (RCC), Hodgkin lymphoma, squamous carcinoma of the head and neck (SCCHN) and bladder cancer [1–6].