PDCD1 and melanoma: Since the first CTLA-4 inhibitor ipilimumab was approved by the US Food and Drug Administration (FDA) for the treatment of metastatic melanoma in 2011, several PD-1 inhibitors (nivolumab, pembrolizumab) and PD-L1 inhibitors (atezolizumab, avelumab, and durvalumab) have been approved by the FDA for the treatment of up to 12 different types of advanced solid tumors: Hodgkin and non-Hodgkin lymphomas, melanoma, Merkel cell carcinoma, and liver, kidney, cervical, head and neck, lung, gastric, colorectal, and bladder cancers [7] (Table 1).